Thin and uniform silver nanowires, method of synthesis and transparent conductive films formed from the nanowires

ABSTRACT

Highly uniform and thin silver nanowires are described having average diameters below 20 nm and a small standard deviation of the diameters. The silver nanowires have a high aspect ratio. The silver nanowires can be characterized by a small number of nanowires having a diameter greater than 18 nm as well as with a blue shifted narrow absorption spectrum in a dilute solution. Methods are described to allow for the synthesis of the narrow uniform silver nanowires. Transparent conductive films formed from the thin, uniform silver nanowires can have very low levels of haze and low values of ΔL*, the diffusive luminosity, such that the transparent conductive films can provide little alteration of the appearance of a black background.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending U.S. patent applicationSer. No. 16/549,204 to Hu et al., filed Aug. 23, 2019, entitled “Thinand Uniform Silver Nanowires, Methods of Synthesis and TransparentConductive Films Formed from the Nanowires,” which is a continuation ofU.S. patent application Ser. No. 16/249,249 to Hu et al., filed Jan. 16,2019, entitled “Thin and Uniform Silver Nanowires, Methods of Synthesisand Transparent Conductive Films Formed from the Nanowires,” which is adivisional of U.S. patent application Ser. No. 15/951,758 to Hu et al.,filed Apr. 12, 2018, entitled “Thin and Uniform Silver Nanowires,Methods of Synthesis and Transparent Conductive Films Formed from theNanowires,” which claims priority to U.S. Provisional Patent ApplicationNo. 62/595,281 to Hu et al., filed Dec. 6, 2017, entitled “Thin andUniform Silver Nanowires, Methods of Synthesis and TransparentConductive Films Formed from the Nanowires,” all of which areincorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to silver nanowires with very small diameters andlarge aspect ratios that can be assembled in useful quantities with veryuniform diameters. The invention further relates to method ofsynthesizing the nanowires, including but not limited to methods basedon particularly desirable catalysts for the synthesis. Also, theinvention relates to transparent conductive films with very low hazewhile achieving high transparency and low electrical resistance.

BACKGROUND OF THE INVENTION

Silver nanowires represent a promising technology for transparentelectrical conductors and other conductor applications as a moreversatile alternative to traditional conductive oxides, such as indiumtin oxide, and other conductive materials. While market penetration ofsilver nanowires for various applications have been limited to date, itis expected that silver nanowires will provide a growing supply ofmaterials for high performance applications. Silver nanowires aregenerally synthesized and subsequently delivered to a structure forincorporating into a product. Thus, the first step of thecommercialization effort involves the synthesis of the silver nanowires.

SUMMARY OF THE INVENTION

In a first aspect, the invention pertains to a collection of nanowirescomprising silver and having an average diameter of no more than about20 nm and a standard deviation of the diameter of no more than about 2.5nm. For thin uniform nanowires, the collection of nanowires can becharacterized by an absorption at 410 nm relative to the maximumabsorption of no more than about 0.225 along with a narrow absorptionpeak when measured in a dilute solution with dimethyl sulfoxide solvent.In some embodiments, no more than about 10% of the nanowires have adiameter greater than 18 nm.

In a further aspect, the invention pertains to a method for synthesizingsilver nanowires, the method comprising forming a reaction solutionsubstantially free of paramagnetic ions comprising a blend of polyolsolvent, polyvinyl pyrrolidone, a chloride salt, and a bromide salt. Thereaction solution comprises a five-membered aromatic heterocyclic cationhaving at least one but no more than three nitrogen atoms and at leastone carbon atom. Suitable five-membered heretocyclic ions include, forexample, imidazolium, pyrazolium, derivatives thereof and mixturesthereof. The reaction solution can be heated to reach a selected peaktemperature, following which the heating may or may not be terminated. Asoluble silver salt is added to the reaction solution. In someembodiments, the soluble silver salt can be added near or after reachingthe peak temperature, e.g. within about 5 degrees C.

In another aspect, the invention pertains to a method for synthesizingsilver nanowires, the method comprising the steps of forming a reactionsolution comprising a blend of polyol solvent, polyvinyl pyrrolidone, asalt comprising chloride and/or bromide, and a neutral organic compoundwith a five-membered aromatic heterocycle comprising at least one but nomore than three nitrogen atoms and at least one carbon atom, heating thereaction solution to a peak temperature, and adding a soluble silversalt. In some embodiments, the soluble silver salt can be added near orafter reaching the peak temperature, e.g. within about 5 degrees C. Theneutral organic compound can be imidazole, pyrazole, derivativesthereof, or mixtures thereof.

In other aspects, the invention pertains to a transparent electricallyconductive structure comprising a transparent substrate and a sparsemetal conductive layer over a surface of the transparent substrate. Inembodiments based in the improved silver nanowires described herein, thetransparent electrically conductive structure can have a sheetresistance of no more than about 100 ohms/sq, a total transmittance ofvisible light of at least about 90% and a haze of no more than about0.60%. In some embodiments, the value of ΔL* obtained in a reflectionconfiguration on a substrate with a black surface is no more than avalue of 2.0, ΔL*=L* of the electrically conductive structure minus L*of the structure without the sparse metal conductive film.

In additional aspect, the invention pertains to a transparentelectrically conductive structure comprising a transparent substrate, afirst sparse metal conductive layer over a first surface of thetransparent substrate, and a second sparse metal conductive layer over asecond surface of the substrate opposite the first surface. In someembodiments, each surface of the transparent electrically conductivestructure has a sheet resistance of no more than about 100 ohms/sq andwherein the transparent electrically conductive structure has a totaltransmittance of visible light of at least about 90% and a haze of nomore than about 0.90%

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transmission electron micrograph (TEM) of silver nanowiresformed using a first set of reactants.

FIG. 2 is a histogram plotting nanowire diameters for the silvernanowires produced using the reaction conditions used to form thenanowires in FIG. 1 .

FIG. 3 is a UV-visible absorption spectrum for the nanowires shown inFIG. 1 .

FIG. 4 is a transmission electron micrograph (TEM) of silver nanowiresformed using a set of reactants comparable to the reactant used to formthe silver nanowires of FIG. 1 except with PVP K90 capping polymer.

FIG. 5 is a histogram plotting nanowire diameters for the silvernanowires produced using the reaction conditions used to form thenanowires in FIG. 4 .

FIG. 6 is a transmission electron micrograph (TEM) of silver nanowiresformed using a set of reactants comparable to the reactant used to formthe silver nanowires of FIG. 1 except with PVP 85N capping polymer.

FIG. 7 is a histogram plotting nanowire diameters for the silvernanowires produced using the reaction conditions used to form thenanowires in FIG. 6 .

FIG. 8 is a UV-visible absorption spectrum for the nanowires shown inFIG. 6 .

FIG. 9 is a transmission electron micrograph (TEM) of silver nanowiresformed using a set of reactants comparable to the reactant used to formthe silver nanowires of FIG. 1 except with a first alternative bromidesalt.

FIG. 10 is a histogram plotting nanowire diameters for the silvernanowires produced using the reaction conditions used to form thenanowires in FIG. 9 .

FIG. 11 is a UV-visible absorption spectrum for the nanowires shown inFIG. 9 .

FIG. 12 is a transmission electron micrograph (TEM) of silver nanowiresformed using a set of reactants comparable to the reactant used to formthe silver nanowires of FIG. 1 except with a second alternative bromidesalt.

FIG. 13 is a histogram plotting nanowire diameters for the silvernanowires produced using the reaction conditions used to form thenanowires in FIG. 13 .

FIG. 14 is a transmission electron micrograph (TEM) of silver nanowiresformed using a set of control reactants.

FIG. 15 is a histogram plotting nanowire diameters for the silvernanowires produced using the reaction conditions used to form thenanowires in FIG. 14 .

FIG. 16 is a transmission electron micrograph (TEM) of silver nanowiresformed using a set of reactants comparable to the control reactants usedto form the silver nanowires of FIG. 14 with the addition of imidazolecatalyst at a first concentration.

FIG. 17 is a histogram plotting nanowire diameters for the silvernanowires produced using the reaction conditions used to form thenanowires in FIG. 16 .

FIG. 18 is a UV-visible absorption spectrum for the nanowires shown inFIG. 16 along with a control spectrum for the silver nanowires shown inFIG. 14 .

FIG. 19 is a transmission electron micrograph (TEM) of silver nanowiresformed using a set of reactants comparable to the control reactants usedto form the silver nanowires of FIG. 14 with the addition of imidazoleat a second concentration.

FIG. 20 is a histogram plotting nanowire diameters for the silvernanowires produced using the reaction conditions used to form thenanowires in FIG. 19 .

FIG. 21 is a UV-visible absorption spectrum for the nanowires shown inFIG. 19 along with a control spectrum for the silver nanowires shown inFIG. 14 .

FIG. 22 is a transmission electron micrograph (TEM) of silver nanowiresformed using a set of reactants comparable to the reactants used to formthe silver nanowires of FIG. 1 with the addition of imidazole at a firstconcentration.

FIG. 23 is a histogram plotting nanowire diameters for the silvernanowires produced using the reaction conditions used to form thenanowires in FIG. 22 .

FIG. 24 is a UV-visible absorption spectrum for the nanowires shown inFIG. 22 along with a control spectrum for the silver nanowirescomparably formed without the imidazole additive.

FIG. 25 is a transmission electron micrograph (TEM) of silver nanowiresformed using a set of reactants comparable to the reactants used to formthe silver nanowires of FIG. 1 with the addition of imidazole at asecond concentration.

FIG. 26 is a histogram plotting nanowire diameters for the silvernanowires produced using the reaction conditions used to form thenanowires in FIG. 25 .

FIG. 27 is a UV-visible absorption spectrum for the nanowires shown inFIG. 25 along with a control spectrum for the silver nanowires withoutthe addition of the imidazole additive.

FIG. 28 is a transmission electron micrograph (TEM) of silver nanowiresformed using a set of reactants comparable to the reactants used to formthe silver nanowires of FIG. 1 with the addition of pyrazole.

FIG. 29 is a histogram plotting nanowire diameters for the silvernanowires produced using the reaction conditions used to form thenanowires in FIG. 28 .

FIG. 30 is a UV-visible absorption spectrum for the nanowires shown inFIG. 28 along with a control spectrum for the silver nanowires withoutthe addition of the imidazole additive.

FIG. 31 is a transmission electron micrograph (TEM) of silver nanowiresformed using a set of reactants comparable to the reactants used to formthe silver nanowires of FIG. 28 .

FIG. 32 is a histogram plotting nanowire diameters for the silvernanowires produced using the reaction conditions used to form thenanowires in FIG. 31 .

FIG. 33 is a UV-visible absorption spectrum for the nanowires shown inFIG. 31 along with a control spectrum for the silver nanowires withoutthe addition of the imidazole additive.

FIG. 34 is a transmission electron micrograph (TEM) of silver nanowiresformed using a set of reactants comparable to the reactants used to formthe silver nanowires of FIG. 28 with a five times volume scale up.

FIG. 35 is a histogram plotting nanowire diameters for the silvernanowires produced using the reaction conditions used to form thenanowires in FIG. 34 .

FIG. 36 is a UV-visible absorption spectrum for the nanowires shown inFIG. 34 along with a control spectrum for the silver nanowires withoutthe addition of the pyrazole additive.

FIG. 37 is a transmission electron micrograph (TEM) of silver nanowiresformed using a set of reactants comparable to the reactants used to formthe silver nanowires of FIG. 1 with the addition of a small amount ofwater to the reactor.

FIG. 38 is a histogram plotting nanowire diameters for the silvernanowires produced using the reaction conditions used to form thenanowires in FIG. 37 .

FIG. 39 is a UV-visible absorption spectrum for the nanowires shown inFIG. 37 along with a control spectrum for the silver nanowires withoutthe addition of the imidazole additive.

FIG. 40 is a schematic sectional view of a structure with conductivelayer comprising a fused metal nanostructured network on a substratewith an overcoat over the conductive layer.

FIG. 41 is a schematic sectional view of structure with a black surfacehaving an optically clear adhesive layer adhering a structure of FIG. 40onto the black surface with an antireflective layer added over theopposite surface of a cyclic olefin substrate.

FIG. 42 is a control structure corresponding to the structure of FIG. 41without the conductive layer or the overcoat layer.

FIG. 43 is a schematic sectional view of a substrate with transparentconductive films applied to opposing surfaces of the substrate with apolymer overcoat applied to the transparent conductive film.

DETAILED DESCRIPTION OF THE INVENTION

Synthesis techniques provide for the production of highly uniform silvernanowires having a very small diameter that can be used for theproduction of transparent conductive films with improved performance. Insome embodiments, the uniform nanowires can have an average diameter nomore than about 20 nm and a uniformity as expressed by a standarddeviation in diameter of no more than about 2.5 nm. The nanowires can befurther characterized by spectroscopic properties of dilute solutions ofthe nanowires, and spectroscopic characterization provide an alternativeapproach to characterize the properties of a collection of thenanowires. The synthesis techniques can be based on the use of both achloride based catalyst and a bromide based catalyst, and in someembodiments at least one of the catalysts may have a catalyst based onfive-membered aromatic heterocyclic cations, such as imidazolium. Inadditional or alternative embodiments, non-ionic five-membered organicheterocyclic compounds, such as imidazole, pyrazole, other isomersthereof, or a derivative thereof, can be also added as a catalyst. Thereaction can be controlled to achieve a small average wire diameter witha high degree of uniformity of the purified silver nanowires. Purifiedsilver nanowires can exhibit the higher degree of uniformity with verysmall average diameters, and can be formulated into dispersions/inks forthe formation of transparent conductive films of highly desirableproperties. In particular, the product transparent conductive films canachieve very low haze and desirably dark reflectivity properties, whichare properties of interest for display applications.

Dispersions of silver nanowires can be deposited on a surface andprocessed into a conductive film. The resulting conductive film can bedesirable due to its mechanical properties, transparency to visiblelight, combinations of these features, or other aspects of theconductive film. In particular, the use of the nanowires to formtransparent conductive films can have significant application in deviceswith displays. While processing of the silver nanowires into the filmscan influence the resulting properties significantly, the use of highquality nanowires can also be a significant factor in the quality of theresulting transparent conducting film. The quality of the nanowires canbe influenced by several factors, but relevant factors include purity ofthe nanowires, thinness of the nanowires, aspect ratio, and as describedherein uniformity of the nanowire size. The thin and uniform silvernanowires described herein can be effective in producing transparentconductive films with very low haze and very low color intensity orluminosity expressed as reflective L* in the CIELAB scale discussedfurther below.

The synthesis approaches described herein have been successful for thesynthesis of particularly thin nanowires with a high degree ofuniformity. In addition, the synthesis approaches have achieved highyields in a format that is amenable to appropriate purificationtechniques to provide for ready commercialization of the resultingsilver nanowires. Due to the highly thin and uniform character of thesilver nanowires, it has been possible to process the nanowires intotransparent conductive films with improved low levels of haze andreflectivity at desired values of electrical conductivity (low sheetresistance) and high transmittance.

While some alternative approaches have been reported for the synthesisof silver nanowires, commercially viable approaches for silver nanowiresynthesis have generally been based on what is generally referred to asthe polyol process, which involves a polyvinylpyrrolidone capping agentand a glycol solvent reducing agent. The first report of the polyolprocess for silver nanowire synthesis based on a polyvinylpyrrolidonecapping agent is generally attributed to Ducamp-Sanguesa et al., Journalof Solid State Chemistry, 100, 272-280 (1992) entitled “Synthesis andCharacterization of Fine and Monodisperse Silver Particles of UniformShape,” incorporated herein by reference. This technique was extended bythe laboratory of Professor Xia, see U.S. Pat. No. 7,585,349 to Xia etal., entitled “Methods of Nanostructure Formation and Shape Selection,”and Wiley et al., “Synthesis of Silver Nanostructures with ControlledShapes and Properties,” Acc. Chem. Res. 2007, 40, 1067-1076, both ofwhich are incorporated herein by reference. Similar synthesis wascarried out with Fe⁺² or Cu⁺² halide salts by Xia et al., “ShapeControlled Synthesis of Metal Nanocrystals: Simple Chemistry MeetsComplex Physics?” Angew. Chem. Int. Ed., 48, 60 (2009). Use of variousother metal halide salts have been used in metal nanowire synthesis. Theuse of quaternary phosphonium salts as a substitute for metal halides isdescribed in U.S. Pat. No. 8,741,025 to Whitcomb, entitled “NanowirePreparation Methods, Compositions, and Articles,” incorporated herein byreference.

Imidazolium halides with a paramagnetic anion have been used ascatalysts for the formation of silver nanowires as discussed in Jo etal., “Synthesis of small diameter silver nanowires via amagnetic-ionic-liquid-assisted polyol process,” RSC Advances 2016, 6,104273-104279. The work of Jo et al., involved a FeCl₄ ⁻ anion. Some oftheir work involved the use of 1-butyl-3-methylimidazoliumtetrachloroferrate (bmim (FeCl₄)) along with bmimBr. The present workwith imidazolium salt catalysts avoids the use of the paramagneticcomponent (FeCl₄) and in some embodiments uses an alternative secondarysalt with a simpler cation in combination with an imidazolium cation.The presence of the iron cation in the reaction solution alters thesynthesis conditions in significant ways that are avoided in the presentwork. The present work also differs from the Jo et al. methods in theaspects of the synthesis conditions. The current work involves theformation of highly uniform nanowires as well as thinner nanowiresrelative to the results of Jo et al.

In general, the reactions are carried out in a liquid polyol solvent,such as ethylene glycol, propylene glycol, combinations thereof, or thelike. A polyvinylpyrrolidone capping agent is used, and variousmolecular weight PVP polymers are exemplified. The use of highermolecular weight polyvinylpyrrolidone to synthesize thinner nanowires isdescribed in da Silva et al., “Facile Synthesis of Sub-20 nm SilverNanowires Through Bromide-Mediated Polyol Method,” ACSNano 2016, 10,7892-7900, incorporated herein by reference.

The uniform, thin silver nanowires described herein have beensynthesized with cations having a five-membered aromatic heterocyclemoiety, and these are described in the context of some specificembodiments. The organic heterocyclic cations can be convenientlyprovided as halides, but other salts with these cations can beeffectively used also, such as nitrates. Whether or not the organicheterocyclic cations are provided as halides, halide anions can beprovided additionally or alternatively with reasonable cations that aresuitable for the reaction conditions, such as Na⁺, K⁺, NH₄ ⁺, mixturesthereof, or the like. Some particularly desirable cationic heterocyclesare discussed first, and a more general discussion of these cationiccatalysts are presented in a later section.

In some embodiments, imidazolium salt compounds, e.g., chloride and/orbromide salts, (1,3-substituted diazacyclopenta-2,4-diene halide) can berepresented by the formula:

where R₁ and R₂ are independently hydrogen, a linear, branched, orcyclic alkyl group, such as methyl, propyl, isopropyl, butyl,cyclohexyl, an unsaturated group such as vinyl, an arylalkyl group suchas benzyl, naphthylmethyl, or other groups with up to at least 30 carbonatoms, with the proviso that both R₁ and R₂ are not both hydrogens, andwhere R₃ is a halogen or any substituent described for R₁ and R₂, andcan be independently located in the 2-, 4-, or 5-positions of thefive-membered ring, and X⁻ is an anion, such as BF₄ ⁻, PF₆ ⁻, Cl⁻ orBr⁻. Some imidazolium-based compounds have a 1-methyl group and a longerchain at the 3 position, although note that the 1 and 3 positions areequivalent due to symmetry. Additional substituents at other ringpositions can also be contemplated. In the reaction solutions herein,the solution lacks a complexing metal cation, which can be expected toalter the equilibrium conditions of the ions in solution. Also, thepresent reaction solutions lack a paramagnetic component. Free chlorideions generally precipitate silver chloride, which can form as smallparticle seeds, due to the very low solubility of silver chloride.Silver bromide also has very low solubility in glycols. Of course inequilibrium, small quantities of both silver chloride and silver bromidedissolve in the glycols according to the respective equilibriumconstants.

Additionally, pyrazolium salts, e.g., halides, are isomeric forms ofimidazolium salts and can be useful catalyst compounds. Collectively,the imidazolium and pyrazolium ions can be referred to as diazolium ionsbased on the five-membered aromatic rings with two nitrogen atoms on thering. A general structure of pyrazolium salt is represented by theformula:

where R₁, R₂, and R₃ are as described above for the imidazolium salts,respectively and can be the same for the pyrazolium salts, and X⁻ is ananion.

Further five-membered aromatic hetrocyclic ions are represented byselected five-membered aromatic heterocycles such as thiazolium salts:

where R₁ and R₂ are respectively as described for the imidazolium saltsand R₃ is hydrogen, a linear, branched, or cyclic alkyl group, such asmethyl, propyl, isopropyl, butyl, cyclohexyl, an unsaturated group suchas vinyl, an arylalkyl group such as benzyl, naphthylmethyl, or othergroups with up to at least 30 carbon atoms, and X⁻ is an anion, such asCl⁻ or Br⁻.

The use of a combination of chloride catalysts and bromide catalysts aredescribed in an article to Hu et al. (hereinafter Hu), entitled“Scalable Coating and Properties of Transparent, Flexible, SilverNanowire Electrodes, ACSNano, Vol. 4(5), (April 2010), 2955-2963,incorporated herein by reference. In some embodiments described herein,imidazolium ions in the catalysts are combined with improved processcontrols to synthesize improved silver nanowires with very small anduniform sizes. Generally, both a chloride salt and bromide salt can beused. Also, for the first time known to Applicant, a non-polymer organicmolecule, imidazole and derivatives thereof, has been found to improvethe geometry of the nanowires.

In combination with ionic catalysts, Applicant has discovered the use ofneutral organic catalysts for further facilitating the synthesis ofthinner highly uniform nanowires. Some examples are presented withsoluble small organic catalyst molecules. For example, imidazolecompound and pyrazole compound (or more generally five-membered aromaticheterocyclic rings with carbon and nitrogen) are shown in the Examplesto contribute to formation of thinner nanowires with selected ioniccatalysts while producing even more uniform nanowires in high yield. Theuse of a non-ionic organic catalyst additive provides as new class ofagents to facilitate the synthesis. The neutral organic catalysts wereused in combination with suitable halide salt catalysts. It is shown tobe possible to form collections of nanowires with a significant fractionof nanowires with diameters below 15 nm.

The Examples below are based on ethylene glycol solvents, althoughpropylene glycol is known in the art to also work for silver nanowiresynthesis as a substitute for or mixed with ethylene glycol. Withrespect to the capping agent, polyvinylpyrrolidone (PVP) and PVPcopolymers have been successfully used. One could imagine other polarpolymers could be substituted for the (PVP) but to date no reports ofcomparable results with other polymer capping agent has been reported ina polyol driven synthesis. PVP copolymers have been used to synthesizerelatively thick silver nanowires as described in published U.S. patent2014/0178247 A1 to Alsayed et al., entitled “Process for Making SilverNanostructures and Copolymer Useful in Such Process,” incorporatedherein by reference. An aqueous synthesis approach without the use of apolymer capping agent is discussed in published U.S. patent application2010/0078197 A1 to Miyagishima et al., entitled “Metal Nanowires,Methods for Producing the Same and Transparent Conductor,” incorporatedherein by reference. Results in the Examples are reported using PVP K30,which has a molecular weight from about 40,000 g/mole to 50,000 g/mol,PVP K85N, which is a purer form of PVP having a molecular weight ofroughly 1,000,000, or PVP K90 having a molecular weight from about900,000-1,600,000 (PVP K90, BASF). The K values of the PVPs relate tocertain viscosity measurements, but these values can generally beconverted to molecular weight ranges. Polymers are generallycharacterized by ranges of molecular weights that are estimated based onaccepted analytical techniques. The distributions of molecular weightscan depend on proprietary synthesis techniques and may vary forparticular products. BASF is believed to be the largest PVP supplier inthe world, but other suppliers are known, such as Ashland ChemicalCompany and Nippon Shokubai. The results with higher molecular weightPVP tended to have smaller diameters, i.e., thinner nanowires, but withsomewhat of a drop in yield.

The catalyst selection along with a higher peak temperature provides fora shorter reaction time while also providing high nanowire yields.Generally, the reactions herein provide improved yields relative toknown comparable synthesis techniques. The high yields and shorterreaction times are desirable features for commercial nanowire synthesis.High reaction temperatures and short reaction times were also used byHu, cited above. But Hu used solid silver chloride catalyst, andexperience of Applicant with these reactants suggests lower reactiontemperatures and longer reaction time to obtain somewhat thinner wires.Using the reactant systems described herein, very thin and uniformsilver nanowires are obtained even with the higher peak reactiontemperature and with shorter reaction times. Based on experience in theart, average silver nanowire diameters have been limited at about 22 nmfor larger scale production yields. Some efforts to produce thinnerwires have succeeded in small reductions in silver nanowire averagediameters, but generally at the expense of uniformity and/or yield. Therespects herein have succeeded in unprecedented thin nanowires whileimproving the diameter uniformity and while obtaining a yield comparableto yields achieved at 22 nm average diameters.

In some embodiments, the improved silver nanowires can have an averagediameter of no more than about 20 nanometers (nm) and generally no morethan about 19 nm. Correspondingly, the nanowires simultaneously have ahigh degree of uniformity as determined both by measurement of diametersin micrographs and spectroscopic measurements, as explained below. Thenanowire diameters can be determined through examination of images fromelectron microscopy. The average is generally evaluated through themeasurement of about 100 representative nanowires. The uniformity indiameters can be conveniently expressed as a standard deviation,although other aspects of the diameter distribution, such as numbers ofwires below specified cutoff values (in particular 18 nm or less) canalso provide useful characterization of the distributions. In someembodiments, the standard deviation is no more than about 3.5 nm. Also,in some embodiments, at least about 75% of the silver nanowires have adiameter less than 18 nm. The average nanowire length can be from about10 microns to about 30 microns. Historically, the nanowire length hasbeen considered a significant parameter, but it has been found that oncea length greater than roughly 10 microns is reached, further lengthincreases do not significantly improve the properties of a transparentconductive layer formed from the nanowires. The UV Visible absorptionspectrum in a suitable dispersant can also be used to characterize thenanowires. A suitable liquid can be dimethyl sulfoxide (DMSO) with asilver nanowire concentration of 0.005 wt %, although at lowconcentrations the normalized spectrum generally is not significantlysensitive to the concentration. As used herein a normalized absorptionspectrum sets the highest value to 1 and the lowest value to 0 withinthe wavelength range of 300 nm to 800 nm. As the average nanowirediameter gets smaller, the absorption maximum tends to shift to lowerwavelengths (blue-shift). As the silver nanowires become more uniform,the absorption peak tends to narrow. Due to changes in peak shape thatare not yet fully understood, there may be additional contributions tothe narrowing of the absorption peak. Nevertheless, for uniform thinsilver nanowires, the absorption peak is narrow and blue shifted, whichcan be measured through the absorption at 410 nm that decreases oralternatively through the absorption peak width.

Transparent electrically conductive elements, e.g., films, of particularinterest herein comprise a sparse metal conductive layer. The conductivelayers are generally sparse to provide desired amount of opticaltransparency, so the coverage of the metal has very significant gapsover and between the layer of the conductive element. For example,transparent electrically conductive films can comprise metal nanowiresdeposited along a layer where sufficient contact can be provided forelectron percolation to provide suitable conduction pathways. The thinand uniform silver nanowires can be effectively used for a range ofstructures providing transparent conducting films involving sparse metalconductive layers. In other embodiments, the transparent electricallyconductive film can comprise a fused metal nanostructured network, whichhas been found to exhibit desirable electrical and optical properties.

In general, various sparse metal conductive layers can be formed frommetal nanowires. Films formed with metal nanowires that are processed toflatten the nanowires at junctions to improve conductivity is describedin U.S. Pat. No. 8,049,333 to Alden et al., entitled “TransparentConductors Comprising Metal Nanowires,” incorporated herein byreferences. Structures comprising surface embedded metal nanowires toincrease metal conductivity are described in U.S. Pat. No. 8,748,749 toSrinivas et al., entitled “Patterned Transparent Conductors and RelatedManufacturing Methods,” incorporated herein by reference. However,improved properties have been found for fused metal nanostructurednetworks with respect to high electrical conductivity and desirableoptical properties with respect to transparency and low haze. Fusing ofadjacent metal nanowires can be performed based on chemical processesunder commercially appropriate processing conditions.

In particular, a significant advance with respect to achievingelectrically conductive films based on metal nanowires has been thediscovery of well controllable processes to form a fused metal networkwhere adjacent sections of the metal nanowires fuse. In particular, itwas discovered that halide ions can drive the fusing of metal nanowiresto form fused metal nanostructures. Fusing agents comprising halideanions were introduced in various ways to successfully achieve thefusing with a corresponding significant drop in the electricalresistance. It should be noted that halide ions in this processingcontext should not be confused with halide ions used in the nanowiresynthesis reactions. Specifically, the fusing of metal nanowires withhalide anions has been accomplished with vapors and/or solutions of acidhalides as well as with solutions of halide salts. Fusing of metalnanowires with halide sources is described further in published U.S.patent application 2013/0341074 to Virkar et al., entitled “MetalNanowire Networks and Transparent Conductive Material,” now U.S. Pat.No. 10,029,916 B2, and U.S. Pat. No. 9,920,207 to Virkar et al. (the'207 patent), entitled “Metal Nanostructured Networks and TransparentConductive Material,” both of which are incorporated herein byreference.

An extension of the process for forming fused metal nanowire networkswas based on reduction/oxidation (redox) reactions that can be providedto result in fused nanowires without destroying the optical propertiesof the resulting film. Metal for deposition at the junctions can beeffectively added as a dissolved metal salt or can be dissolved from themetal nanowires themselves. The effective use of redox chemistry forfusing metal nanowires into a nanostructured network is describedfurther in published U.S. patent application 2014/0238833A1 to Virkar etal. (the '833 application), entitled “Fused Metal NanostructuredNetworks, Fusing Solutions with Reducing Agents and Methods for FormingMetal Networks,” now U.S. Pat. No. 10,020,807 B2, incorporated herein byreference. The '833 application also described a single solutionapproach for the formation of fused metal nanostructured networks.Single solution approaches for the formation of fused metalnanostructured layers are described further in U.S. Pat. No. 9,183,968B1 to Li et al, (hereinafter the '968 patent) entitled “Metal NanowireInks for the Formation of Transparent Conductive Films with FusedNetworks,” incorporated herein by reference, and single solution or inkprocessing to form fused metal nanostructured networks is used in theExamples below.

The desirable inks to achieve effective single deposition inks that cureinto fused nanostructured metal networks comprise a desired amount ofmetal nanowires to achieve appropriate loading of metal in the resultingfilm. In appropriate solutions, the inks are stable prior to depositionof the ink and drying. The inks can comprise a reasonable amount ofpolymer binder that contributes to the formation of a stable conductingfilm for further processing. To obtain good fusing results with one inksystems, hydrophilic polymers have been found to be effective, such ascellulose or chitosan based polymers. Metal ions, as a source of metalfor the fusing process, can be supplied as a soluble metal salt.

A single ink formulation provides for depositing a desired loading ofmetal as a film on the substrate surface and simultaneously providingconstituents in the ink that induce the fusing process as the ink isdried under appropriate conditions. These inks can be referred toconveniently as fusing metal nanowire inks with the understanding thatthe fusing generally does not take place until drying. The inksgenerally comprise an aqueous solvent, which can further comprise analcohol and/or other organic solvent in some embodiments. The inks canfurther comprise dissolved metal salts as a metal source for the fusingprocess. Without wanting to be limited by theory, it is believed thatcomponents of the ink, e.g., alcohol, or other organic compositions,reduce the metal ions from solution to drive the fusing process.Previous experience with the fusing process in these systems suggeststhat the metal preferentially deposits at the junctions between adjacentmetal nanowires. A polymer binder can be provided to stabilize the filmand to influence ink properties. The particular formulation of the inkcan be adjusted to select ink properties suitable for a particulardeposition approach and with specific coating properties on a substratesurface. As described further below, drying conditions can be selectedto effectively perform the fusing process. Two solution fusing systemshave also been established and can take advantage of the improved silvernanowires described herein.

The fused metal nanostructured networks have been found to provide forlow sheet resistances while correspondingly providing high transmittanceand low haze. Reducing the nanowire thickness can further improve theoptical properties. The results herein also demonstrate that theuniformity of nanowires also contributes to further improvement of theoptical properties. In particular, it is found that a reduced number ofnanowires with a diameter of 18 nm or greater provides a very low hazevalue and a very low value of diffuse reflective L*. The synthesistechniques described herein can be selected to produce silver nanowireswith a uniformity that corresponds with over 75% of the nanowires havinga diameter of no more than 18 nm. In some embodiments, the value of L*in a reflective configuration attributable to the transparent conductivefilm can be no more than about 2.0, where this parameter is describedbelow as ΔL* in which the L* of the other structural features issubtracted from the overall value of L*. Values of haze in atransmission configuration can be no more than about 0.6% for fusedmetal nanostructured networks at values of sheet resistance of no morethan about 100 ohms/sq formed with the thinner, uniform silvernanowires.

Silver Nanowire Synthesis and Purification

Improved silver nanowires synthesis has achieved highly uniform silvernanowires with small average diameters. The synthesis is suitable forlarge scale synthesis at relatively high yield. Improve catalysts andcombinations of catalysts with desired short reaction times can providethe highly uniform nanowires. In particular, desirable results aredescribed with dialkyl imidazolium halides in salt blends with achloride salt and a bromide salt, which may comprise an inorganic cationalso. Further surprisingly improved results are obtained with smallmolecule organic catalysts, such as imidazole, and may be expectedsimilarly from isomers thereof. The selection of the PVP capping polymercan also favorably influence the silver nanowire thinness. Generally, apurification step is necessary to isolate the nanowires from thereaction mixture comprising significant amount of other nanostructuressuch as nanoparticles. Concentrations referred to in this section arebased on amounts added to the reaction solution and the volume, and theactual concentrations in solution evolve based on the interactions andthe reactions of the species in solution.

In the polyol processes, the solvent is a diol, generally ethyleneglycol, propylene glycol or a blend thereof. The diols can function as areducing agent with controllable properties, such as through theapplication of heat. While being the solvent, the amount of glycolgenerally is set based on the scale of the reaction, and the otherreactants are adjusted accordingly. The standard silver salt source forproviding the silver ions to be formed into the nanowires is silvernitrate, which is soluble in the glycols, although other soluble silversalts could be used in principle. The soluble silver salt is added todrive the nanowire synthesis. Since the silver is substantially allconsumed, the concentration of silver can change significantly over thecourse of the reaction. Generally, the silver nitrate may be added as asolid or dissolved in glycol solvent, and furthermore the silver nitratecan be added substantially all at once or gradually. In the Examplesherein, a silver nitrate solution in ethylene glycol is addedsubstantially all at once, which for practical purposes is over thecourse of no more than about 10 minutes.

Polyvinylpyrrolidone has been used in the polyol process as a cappingagent. While not wanting to be limited by theory, it is believed thatpolyvinylpyrrolidone (PVP) preferentially associates with certaincrystal lattices of crystalline silver such that silver deposits thenalong the other facets of the crystal to form the nanowires. The work ofXia's research group has examined the synthesis of various silvernanostructures. The molecular weight of the PVP can influence thesynthesis reaction. PVP K30, having a molecular weight from about40,000-60,000 g/mol, can be successfully used to form the thinnanowires. PVP K-90 (or K-85) with molecular weights of 900,000 to1,600,000 or blends with PVP K30 have also been successfully used toobtain similar results as presented herein. With other parameters beingequal, the higher molecular weight PVP capping agents tend to formsomewhat thinner nanowires but as presently found at a significantreduction of yield. The reaction mixture generally comprises from about0.1 weight percent to about 10 wt %, in further embodiments from about0.2 wt % to about 9 wt %, and in other embodiments from about 0.25 wt %to about 8 wt % PVP in the reaction mixture. A person of ordinary skillin the art will recognize that additional ranges of PVP concentrationswithin the explicit ranges above are contemplated and are within thepresent disclosure.

A soluble silver salt is added to provide silver ions for chemicalreduction to silver that is assembled into the nanowires. As a practicalmatter, silver nitrate (AgNO₃) has been the only readily availablesilver salt that is suitably soluble in the glycol solvents. To theextent that other more esoteric silver salts could be identified withappropriate solubility, it is expected that they would be costprohibitive relative to silver nitrate. With respect to silver nitrate,the reaction mixture generally comprises from about 0.0025M to about0.25M, in further embodiments from about 0.005 to about 0.20M, and inother embodiments from about 0.01M to about 0.15M silver nitrate in thereaction mixture, and other soluble silver salts could be added toachieve comparable molar concentrations. A person of ordinary skill inthe art will recognize that additional ranges of soluble silverconcentrations within the explicit ranges above are contemplated and arewithin the present disclosure. As silver nitrate is converted over thecourse of the reaction, an indication of the reaction mixture“comprising” a certain amount of silver nitrate refers to the relativeamount of silver nitrate added over the course of reaction rather thannecessarily the amount of silver in solution at a particular time, whichis in flux and not readily measurable.

The use of specific blends of catalysts salts has resulted in theimproved nanowire syntheses described herein. Generally, the reactionsolution comprises a chloride salt and a bromide salt, so that at leasttwo salts are mixed into the glycol solvent. To obtain the desirablesilver nanowire properties, the reaction solution is substantially freeof paramagnetic ions, such as iron ions. In the context of thisapplication, substantially free refers to no added ions and generally nomore than 1×10⁻⁶M paramagnetic ions. In some embodiments, the desirableionic catalysts comprise a molar ratio of bromide concentration dividedby chloride concentration of the catalysts as added from about 0.5 toabout 15, in further embodiments from about 0.75 to about 10, inadditional embodiments from about 0.9 to about 7 and in otherembodiments from about 0.95 to about 6. A person of ordinary skill inthe art will recognize that additional ranges of ion ratios within theexplicit ranges above are contemplated and are within the presentdisclosure.

With respect to the cations, the salts generally comprise an organiccation, such as imidazolium, and in some embodiments a blend of cationscan be effectively used. If two distinct cations are used, one of thecations can be an inorganic cation, such as alkali metal cations, suchas Na⁺ or K⁺, ammonium cation NH₄ ⁺, substituted ammonium cations, suchas tetramethyl ammonium, or hexyl trimethyl ammonium, or mixturesthereof. Some successful results are obtained in the examples with ablend of an imidazolium chloride salt and imidazolium bromide salt, anda further optional non-ionic catalyst can be included and/or a blend oforganic cations as described herein. Organic cations can be deliveredwith alternative anions, such as nitrate, as long as desired chlorideand/or bromide anions are provided. In general, organic cations ofparticular interest for thin nanowire synthesis comprise a five-memberedaromatic heterocycle with from one to three nitrogen atoms on thearomatic ring and one or more carbons along with an optional additionalheteroatom, such as oxygen or sulfur. Generally, the catalyst salts canindividually be at concentrations from about 0.00001M to about 0.01M, infurther embodiments from about 0.00002M to about 0.005M, and in otherembodiments from about 0.00005M to about 0.0025M. A person of ordinaryskill in the art will recognize that additional ranges of concentrationwithin these explicit ranges are contemplated and are within the presentdisclosure.

The organic cations with a five-membered aromatic heterocycle includemoieties based on imidazolium, pyrazolium, thiazolium, oxazolium,triazolium and the like. Suitable imidazolium cations include, forexample, 1-butyl-3-methylimidazolium, 1-ethyl-3-methylimidazolium,1-hexyl-3-methyl imidazolium, 1-octyl-3-methylimidazolium,1,3-di-isopropylimidazolium, 1,3-dicyclohexylimidazolium, or the like,or mixtures thereof. Similar derivatives can be used for the otherheterocycles. Examples presented below are performed with1-octyl-3-methylimidazolium chloride, but some preliminary experimentswith these other enumerated cations have yielded reasonably comparableresults under the conditions of those experiments.

In some embodiments, a non-ionic organic catalyst can also be added.Examples are presented with imidazole or pyrazole were added along withchloride and bromide salt catalyst additives to provide desirably thinand uniform nanowires. However, the non-ionic catalysts can be usefulwith only a single halide salt catalyst, such as either a chloride or abromide as well as with reaction solutions having both a chloride saltand a bromide salt. For embodiments in which a single halide salt isused along with the non-ionic catalysts, the range of halide saltconcentrations would still be within the values presented above and thegeneral reaction conditions for the synthesis with the non-ioniccatalysts overlap with the reaction conditions for the synthesisreactions with just the ionic salt catalysts.

It is found that aromatic heterocycles such as imidazole promoteformation of thinner nanowires at comparable or better uniformity andhigh yield. In particular, five-membered aromatic heterocyclic ringswith carbon and nitrogen can be used as non-ionic catalysts. Thefive-membered aromatic heterocycles can have from 1 to 3 nitrogen atomsand at least one carbon atom, and can include other atoms, such asthiazole or oxazole. In addition to imidazole, substituted imidazolescan also be used as catalysts, such as 2-methylimidazole,4(5)-(hydroxymethyl)imidazole, 4-isopropylimidazole,4(5)-bromo-5(4)-methyl-imidazole, 2-bromo-1H-imidazole, 1-vinylimidazoleand oligomers thereof, or the like or mixtures thereof. Pyrazole andsubstituted pyrazoles such as 3-cyclopropyl-1H-pyrazole,5-methyl-1H-pyrazole, 3-(trifluoromethyl) pyrazole, and1,4,5,6-tetrahydrocyclopenta[c]pyrazole may also be useful catalysts.Unsubstituted or substituted oxazoles such as oxazole-5-methanol,pyrrole and substituted pyrrole, and thiazole and substituted thiazolessuch as 2-methyl-1,3-thiazole may also be useful catalysts as isomers ofimidazole and pyrazole. The vinyl derivatives can be polymerized to formdimers and larger oligomers or polymers, which would be expected tomaintain their catalytic ability. Other derivatives include, forexample, bicyclic derivatives, such as purine, which has an aromaticimidazole ring adjoined with a pyrimidine heterocyclic ring. In thereaction mixture, the organic non-ionic catalysts can have aconcentration range from about 0.0001M to about 0.1M, in further rangesfrom 0.00025M to about 0.025M and in other embodiments from about0.0005M to about 0.02M. A person of ordinary skill in the art willrecognize that additional ranges of concentration within the explicitranges above are contemplated and are within the present disclosure.

To control the as-synthesized nanowire diameters at smaller values, ithas been found that addition of a small amount of water to the reactioncan reduce the nanowire diameter somewhat without broadening or slightlyimproving the diameter distribution. The water can be added prior to theaddition of the silver nitrate. Generally, deionized water can be usedalthough various available grades of purified water can be suitable. Insome embodiments, the reaction solution can comprise from about 0.01 toabout 5 wt % water, in further embodiments from about 0.025 to about 4wt %, and in additional embodiments from about 0.05 to about 2 wt %water at the start of reaction. The reaction can be carrier forward inthe same way as if there was no water. A person of ordinary skill in theart will recognize that additional ranges of water amounts within theexplicit ranges above are contemplated and are within the presentdisclosure.

The synthesis procedure starts with a stirred reactor of an appropriatesize. Generally, the reaction can be successful with a closed system oran open system, but a closed system can be desirable to achieve betterreproducibility. For the syntheses described herein, the closed systemsare not purged, and the system is open to ambient air through acondenser. A significant portion of the solvent is generally added atthe start of the reaction along with the PVP, and heat is introduced toincrease the solvent temperature. Heating can be performed, for example,with a heating mantle. Stirring can be initiated using either amechanical stirrer or a magnetic stirrer. Heating is continued to reachthe target temperature. When the reaction vessel reaches the target peaktemperature, the heat may or may not be discontinued and the coolingrate can be adjusted in part based on the desired reaction time. Thetarget peak temperature can be at least about 130° C., in furtherembodiments at least about 135° C., in other embodiments at least about140° C., and in additional embodiments from about 145° C. to one degreebelow the solvent boiling point for any of these ranges. A person ofordinary skill in the art will recognize that additional ranges oftemperature within the explicit ranges above are contemplated and arewithin the present disclosure.

Prior to addition of silver nitrate to start the synthesis reaction, thecatalysts can be added sequentially or together. In general, thecatalysts, salt catalysts and/or neutral catalysts, can be added assolids, liquid salts, solutions, or combinations thereof. The pluralityof catalysts can be added sequentially or simultaneously or acombination thereof. Following stirring of the catalysts and at or nearthe peak target temperature, the soluble silver salt, generally silvernitrate, is added. The silver salt can be added as a solid or glycolsolution. Especially if the catalysts and silver salt are added assolids but even in other embodiments, additional glycol, that can bereferred to as top off solvent, can be added to reach the total desiredvolume for the reaction mixture. In some embodiments, from about 2percent to about 40 percent of the total solvent can be added as top offsolvent, which can be added at the same time as, following, or a portionof which at the same time and a portion of which following addition ofthe catalysts, which can complete the formation of the reaction mixture,although in additional embodiments the full amount of solvent can beadded with the dissolved silver salt. Stirring is continued until thereaction is completed. From the start of heating the reactor chargedwith solvent until completing the reaction can be from about 1 hour (hr)to about 10 hrs, in further embodiments from about 1.75 hrs to about 8hrs, and in other embodiments from about 1.9 hrs to about 3.5 hrs. Aperson of ordinary skill in the art will recognize that additionalranges of time within the explicit ranges are contemplated and arewithin the present disclosure.

Following completion of the reaction, the completed reaction mixture isready for purification following cooling. Any large structures in thereactor following the synthesis can be removed through filtrations,centrifugation or other appropriate processes. Following any basicpurification steps to remove large particles, silver nanowires havegenerally been further purified through destabilization of thedispersion and collecting the settled nanowires, such as aftercentrifugation. Destabilization directed to silver nanowire purificationcan be effectuated through the addition of acetone or similar organicsolvent miscible with the glycol but in which the nanowires are lessstably dispersible. Following destabilization of the dispersion,centrifugation can be performed to collect the nanowires, while smallsilver particles may remain dispersed. The process can be repeated tofurther improve the purification.

Following purification, the yield of the reaction can be evaluated. Todetermine the yield, the weight of the purified and harvested nanowiresis divided by the weight of the silver provided with the soluble silversalt. In the reactions described herein, the yield can be at least about10%, in further embodiments, at least about 12%, and in otherembodiments from about 15% to about 35%. A person of ordinary skill inthe art will recognize that additional ranges within the explicit rangesabove are contemplated and are within the present disclosure.

It has been found that a noble metal, such as gold or platinum, can becoated over the silver nanowires to improve the stability and chemicalinertness of the structures. Effective processes have been developed forperforming the coatings to form thin uniform coatings that do notsignificant alter the optical properties of structures formed with thecoated nanowires. The coating of the nanowires is described further inU.S. Pat. No. 9,530,534 to Hu et al., entitled “Transparent ConductiveFilm,” incorporated herein by reference. The nanowires can have a noblemetal coating from about 0.05 wt % to about 15 wt % noble metal relativeto the entire nanowire weight. A person of ordinary skill in the artwill recognize that additional ranges of deposition amounts within theexplicit ranges above are contemplated and are within the presentdisclosure.

In one method of applying the noble metal coating, a method for formingnoble metal coated silver nanowires by direct metal deposition comprisesgradually adding a coating solution comprising dissolved noble metalions and a metal ion complexing ligand into a reaction solutioncomprising silver nanowires and a reducing agent to form the noble metalcoating on the silver nanowires. The reaction solution can furthercomprise a capping polymer, such as PVP (polyvinylpyrrolidone). In analternative method for applying the noble metal coating, a method forforming noble metal coated silver nanowires by galvanic exchangecomprises gradually adding a coating solution comprising a mixture ofnoble metal ions and complexing ligands to a heated reaction solutioncomprising a dispersed blend of silver nanowires and a polymer cappingagent to gradually replace silver with noble metal to form an intactsilver core with a noble metal coating.

Silver Nanowire Characterization

The purified nanowires can be characterized using electron microscopyand the absorption spectrum of a dispersion of the nanowires.Transmission electron micrographs can be used to measure the nanowirediameters. The lengths of the nanowires can be measured using scanningelectron micrographs or by using a good quality optical microscope.Based on the magnification, an electron microscope image has anassociated scale. Using the scale, the image can be directly used tomake measurements of the diameters and lengths.

A randomly selected set of roughly 100 nanowires or more can generallybe used, although reasonably accurate results can be obtained with fewernanowires. For the uniform nanowires described herein, the reportednumbers based on 100 or more nanowires are believed accurate roughly to0.1 nm for diameters based on the measurements. The average diametersfor the uniform silver nanowires can be no more than about 20 nm, inother embodiments no more than about 19 nm, in some embodiments no morethan about 18 nm, in further embodiments no more than about 17.5 nm, inadditional embodiments no more than about 17.0 nm, and in otherembodiments from about 12 to about 19.0 nm. With respect to lengths, thesilver nanowires can have average lengths from about 5 microns to about30 microns. An aspect ratio can be specified as the ratio of the averagelength divided by the average diameter. In some embodiments, thenanowires can have an average aspect ratio of at least about 400 and infurther embodiments from about 500 to about 10,000. A person of ordinaryskill in the art will recognize that additional ranges of silvernanowire dimensions within the explicit ranges above are contemplatedand are within the present disclosure.

With respect to characterizing the nanowire diameter distribution,several methods seem useful. For example, the percent of nanowires withdiameters below 18 nm or below 15 nm can be provided. For someapplications, it can be desirable to not have too large of a percentageof larger nanowires and this provided further information on thediameter distribution. In some embodiments, the silver nanowires canhave at least about 60% with a diameter no more than 18 nm, in furtherembodiments, at least about 65%, in other embodiments at least about 75%and in additional embodiments at least about 85% of the silver nanowireswith a diameter of no more than 18 nm. As noted below, an elimination ofmost of the nanowires with a diameter above 18 nm can provide for theformation of transparent conductive films with particularly low valuesof haze and diffusive luminosity (L*). Furthermore, the silver nanowirescan have at least about 2% with a diameter of no more than 15 nm, infurther embodiments at least about 15%, and in other embodiments atleast about 40% of the nanowires have a diameter no more than 15 nm. Asnoted above, the standard deviation provides significant informationregarding the uniformity of the nanowire diameters. The standarddeviation (SD) in diameter is calculate as,SD=(Σ(d_(n)−d)²)^(1/2)/(N−1), where N is the number of measureddiameters, n=1 to N which is summed over as indicated by Σ, and d is theaverage diameter. In some embodiments, the standard deviation is no morethan about 3.5 nm, in further embodiments no more than 3.0 nm, in otherembodiments no more than about 2.5 nm and in additional embodiments nomore than about 2.0 nm. A person of ordinary skill in the art willrecognize that additional ranges of uniformity measures within theexplicit ranges above are contemplated and are within the presentdisclosure.

The nanowires can be dispersed in a suitable solvent for obtaining theabsorption spectra, which can be an independent approach to characterizethe silver nanowire size and uniformity. Dimethyl sulfoxide (DMSO) is adesirable solvent for measuring the spectrum of the nanowires sincesilver nanowires are dispersible in the DMSO and since DMSO has lowabsorption within the region of interested (300 to 500 nm) and DMSO hasa relatively high refractive index. DMSO is a commonly used solvent forUV-visible absorption studies, and references to the silver nanowirespectrum will be referenced to spectrum taken in dilute DMSOdispersions. In general, the normalized absorption spectrum should beindependent of concentrations for dilute dispersions. Other polarsolvents like alcohols and water can also be used, but these solventshave lower refractive indices compared to DMSO. In general, thecorresponding spectrum of the silver nanowires in water would beexpected to be sharper and slightly blue shifted relative to the spectrain DMSO.

As a general rule, it has been found that thinner silver nanowiresexhibit a peak absorption spectrum between 350 nm and 400 nm that shiftsto lower wavelengths as the silver nanowires become thinner and moreuniform. For spectrum taken in dilute DMSO dispersions, the absorptionspectrum can have a peak below 376 nm and in some embodiments below 374nm. Normalized absorptions can be readily used for evaluating theabsorption spectrum to eliminate significant scaling complexitiesinvolved with the measurements. As used herein, the spectrum between 300nm and 800 nm is normalized to have absorbance values between 0 and 1.Similarly, more uniform silver nanowire diameters can exhibit a narrowerabsorption peak, and this narrower peak can be characterized by thenormalized absorption at 410 nm, which is along the higher wavelengthdecreasing edge of the absorption peak. So a smaller normalizedabsorption at 410 nm is generally associated with a narrower absorptionpeak and a correspondingly more uniform silver nanowire diameters. Insome embodiments, the normalized absorption at 410 nm can be no morethan 0.20 and in further embodiments no more than about 0.185. A personof ordinary skill in the art will recognize that additional ranges ofnormalized absorption values within the explicit ranges above arecontemplated and are within the present disclosure. The resultspresented herein are generally consistent with this qualitativeunderstanding, but the absorption spectra are not yet fully understoodin terms of the silver nanowire distributions.

Sparse Metal Conductive Layers

Sparse metal conductive layers are generally formed from metalnanowires. With sufficient loading and selected nanowire properties,reasonable electrical conductivity can be achieved with the nanowireswith corresponding appropriate optical properties. It is expected thattransparent conductive film structures formed with the thin, uniformsilver nanowires described herein can yield desirable performance forfilms with various sparse metal conductive structures. However,particularly desirable properties have been achieved with fused metalnanostructured networks. Alternative embodiments can be provided inwhich fusing components are not included in the process solutions, andsatisfactory results can be achieved generally with the unfused filmswith the thin, uniform silver nanowires contributing to relativeimprovements in properties. The discussion herein focuses on embodimentsinvolving fused metal nanostructured networks that have been found toprovide improved performance properties as transparent conductive films.

Several practical approaches have been developed to accomplish the metalnanowire fusing. The metal loading can be balanced to achieve desirablelevels of electrical conductivity with good optical properties. Theoptical properties for the conductive films can be improved at aparticular level of electrical conductivity through the use of the thin,uniform silver nanowires described herein. In general, the metalnanowire processing can be accomplished through deposition of two inkswith the first ink comprising the metal nanowires and the second inkcomprising a fusing composition, or through the deposition of an inkthat combines the fusing elements into the metal nanowire dispersion.The inks may or may not further comprise additional processing aids,binders or the like. Suitable patterning approaches can be selected tobe suitable for the particular ink system.

In general, one or more solutions or inks for the formation of the metalnanostructured network can collectively comprise well dispersed metalnanowires, a fusing agent, and optional additional components, forexample, a polymer binder, a crosslinking agent, a wetting agent, e.g.,a surfactant, a thickener, a dispersant, other optional additives orcombinations thereof. The solvent for the metal nanowire ink and/or thefusing solution if distinct from the nanowire ink can comprise anaqueous solvent, an organic solvent or mixtures thereof. In particular,suitable solvents include, for example, water, alcohols, ketones,esters, ethers, such as glycol ethers, aromatic compounds, alkanes, andthe like and mixtures thereof. Specific solvents include, for example,water, ethanol, isopropyl alcohol, isobutyl alcohol, tertiary butylalcohol, methyl ethyl ketone, glycol ethers, methyl isobutyl ketone,toluene, hexane, ethyl acetate, butyl acetate, ethyl lactate, PGMEA(2-methoxy-1-methylethylacetate), dimethyl carbonate, or mixturesthereof. While the solvent should be selected based on the ability toform a good dispersion of metal nanowires, the solvents should also becompatible with the other selected additives so that the additives aresoluble in the solvent. For embodiments in which the fusing agent isincluded in a single solution with the metal nanowires, the solvent or acomponent thereof may or may not be a significant component of thefusing solution, such as alcohols and can be selected accordingly ifdesired.

The metal nanowire ink, in either a one ink or two ink configuration,can include from about 0.01 to about 1 weight percent metal nanowires,in further embodiments from about 0.02 to about 0.75 weight percentmetal nanowires and in additional embodiments from about 0.04 to about0.5 weight percent metal nanowires. A person of ordinary skill in theart will recognize that additional ranges of metal nanowireconcentrations within the explicit ranges above are contemplated and arewithin the present disclosure. The concentration of metal nanowiresinfluences the loading of metal on the substrate surface as well as thephysical properties of the ink.

Silver provides excellent electrical conductivity, and commercial silvernanowires are available. However, thin and uniform silver nanowiressynthesized using the processes herein provide improved transparentconductive films. The properties of these nanowires are summarized indetail above and in the Examples below.

Polymer binders and the solvents are generally selected consistentlysuch that the polymer binder is soluble or dispersible in the solvent.In appropriate embodiments, the metal nanowire ink generally comprisesfrom about 0.02 to about 5 weight percent binder, in further embodimentsfrom about 0.05 to about 4 weight percent binder and in additionalembodiments from about 0.1 to about 2.5 weight percent polymer binder.In some embodiments, the polymer binder comprises a crosslinkableorganic polymer, such as a radiation crosslinkable organic polymerand/or a heat curable organic binder. To facilitate the crosslinking ofthe binder, the metal nanowire ink can comprise in some embodiments fromabout 0.0005 wt % to about 1 wt % of a crosslinking agent, in furtherembodiments from about 0.002 wt % to about 0.5 wt % and in additionalembodiments from about 0.005 wt % to about 0.25 wt %. The nanowire inkcan optionally comprise a rheology modifying agent or combinationsthereof. In some embodiments, the ink can comprise a wetting agent orsurfactant to lower the surface tension, and a wetting agent can beuseful to improve coating properties. The wetting agent generally issoluble in the solvent. In some embodiments, the nanowire ink cancomprise from about 0.001 weight percent to about 1 weight percentwetting agent, in further embodiments from about 0.002 to about 0.75weight percent and in other embodiments from about 0.003 to about 0.6weight percent wetting agent. A thickener can be used optionally as arheology modifying agent to stabilize the dispersion and reduce oreliminate settling. In some embodiments, the nanowire ink can compriseoptionally from about 0.05 to about 5 weight percent thickener, infurther embodiments from about 0.075 to about 4 weight percent and inother embodiments from about 0.1 to about 3 weight percent thickener. Aperson of ordinary skill in the art will recognize that additionalranges of binder, wetting agent and thickening agent concentrationswithin the explicit ranges above are contemplated and are within thepresent disclosure.

A range of polymer binders can be suitable for dissolving/dispersing ina solvent for the metal nanowires, and suitable binders include polymersthat have been developed for coating applications. Hard coat polymers,e.g., radiation curable coatings, are commercially available, forexample, as hard coat materials for a range of applications, that can beselected for dissolving in aqueous or non-aqueous solvents. Suitableclasses of radiation curable polymers and/or heat curable polymersinclude, for example, polyurethanes, acrylic resins, acrylic copolymers,cellulose ethers and esters, other water insoluble structuralpolysaccharides, polyethers, polyesters, epoxy containing polymers, andmixtures thereof. Examples of commercial polymer binders include, forexample, NEOCRYL® brand acrylic resin (DMS NeoResins), JONCRYL® brandacrylic copolymers (BASF Resins), ELVACITE® brand acrylic resin (LuciteInternational), SANCURE® brand urethanes (Lubrizol Advanced Materials),cellulose acetate butyrate polymers (CAB brands from Eastman™ Chemical),BAYHYDROL™ brand polyurethane dispersions (Bayer Material Science),UCECOAT® brand polyurethane dispersions (Cytec Industries, Inc.),MOWITOL® brand polyvinyl butyral (Kuraray America, Inc.), celluloseethers, e.g., ethyl cellulose or hydroxypropyl methyl cellulose, otherpolysaccharide based polymers such as chitosan and pectin, syntheticpolymers like polyvinyl acetate, and the like. In particular,polysaccharide based polymers has been found to be desirable as bindersin sparse metal conductive layers. The polymer binders can beself-crosslinking upon exposure to radiation, and/or they can becrosslinked with a photoinitiator or other crosslinking agent. In someembodiments, photocrosslinkers may form radicals upon exposure toradiation, and the radicals then induce crosslinking reactions based onradical polymerization mechanisms. Suitable photoinitiators include, forexample, commercially available products, such as IRGACURE® brand(BASF), GENOCURE™ brand (Rahn USA Corp.), and DOUBLECURE® brand (DoubleBond Chemical Ind., Co, Ltd.), combinations thereof or the like.

Wetting agents can be used to improve the coatability of the metalnanowire inks as well as the quality of the metal nanowire dispersion.In particular, the wetting agents can lower the surface energy of theink so that the ink spreads well onto a surface following coating.Wetting agents can be surfactants and/or dispersants. Surfactants are aclass of materials that function to lower surface energy, andsurfactants can improve solubility of materials. Surfactants generallyhave a hydrophilic portion of the molecule and a hydrophobic portion ofthe molecule that contributes to its properties. A wide range ofsurfactants, such as nonionic surfactants, cationic surfactant, anionicsurfactants, zwitterionic surfactants, are commercially available. Insome embodiments, if properties associated with surfactants are not anissue, non-surfactant wetting agents, e.g., dispersants, are also knownin the art and can be effective to improve the wetting ability of theinks. Suitable commercial wetting agents include, for example, COATOSIL™brand epoxy functionalized silane oligomers (Momentum PerformanceMaterials), SILWET™ brand organosilicone surfactant (MomentumPerformance Materials), THETAWET™ brand short chain non-ionicfluorosurfactants (ICT Industries, Inc.), ZETASPERSE® brand polymericdispersants (Air Products Inc.), SOLSPERSE® brand polymeric dispersants(Lubrizol), XOANONS WE-D545 surfactant (Anhui Xoanons Chemical Co.,Ltd), EFKA™ PU 4009 polymeric dispersant (BASF), MASURF FP-815 CP,MASURF FS-910 (Mason Chemicals), NOVEC™ FC-4430 fluorinated surfactant(3M), mixtures thereof, and the like.

Thickeners can be used to improve the stability of the dispersion byreducing or eliminating settling of the solids from the metal nanowireinks. Thickeners may or may not significantly change the viscosity orother fluid properties of the ink. Suitable thickeners are commerciallyavailable and include, for example, CRAYVALLAC™ brand of modified ureasuch as LA-100 (Cray Valley Acrylics, USA), polyacrylamide, THIXOL™ 53Lbrand acrylic thickener, COAPUR™ 2025, COAPUR™ 830W, COAPUR™ 6050,COAPUR™ XS71 (Coatex, Inc.), BYK® brand of modified urea (BYKAdditives), Acrysol DR 73, Acrysol RM-995, Acrysol RM-8W (Dow CoatingMaterials), Aquaflow NHS-300, Aquaflow XLS-530 hydrophobically modifiedpolyether thickeners (Ashland Inc.), Borchi Gel L 75 N, Borchi Gel PW25(OMG Borchers), and the like.

Additional additives can be added to the metal nanowire ink, generallyeach in an amount of no more than about 5 weight percent, in furtherembodiments no more than about 2 weight percent and in furtherembodiments no more than about 1 weight percent. Other additives caninclude, for example, anti-oxidants, UV stabilizers, defoamers oranti-foaming agents, anti-settling agents, viscosity modifying agents,or the like.

In some embodiments, a process is used in which a sparse nanowire filmis initially deposited, and further processing may or may not involvethe fusing of the silver nanowires. As noted above, the fusing into aunitary fused metal nanostructured network can provide improvedproperties of the transparent conductive film. For the deposition of themetal nanowire ink, any reasonable deposition approach can be used, suchas dip coating, spray coating, knife edge coating, bar coating,Meyer-rod coating, slot-die coating, gravure printing, spin coating orthe like. The ink can have properties, such as viscosity, adjustedappropriately with additives for the desired deposition approach.Similarly, the deposition approach directs the amount of liquiddeposited, and the concentration of the ink can be adjusted to providethe desired loading of metal nanowires on the surface. After forming thecoating with the dispersion, the sparse metal conductive layer can bedried to remove the liquid.

Fusing of the metal nanowires can be accomplished through variousagents. Without wanting to be limited by theory, the fusing agents arebelieved to mobilize metal ions, and the free energy seems to be loweredin the fusing process. Excessive metal migration or growth may lead insome embodiments to a degeneration of the optical properties, sodesirable results can be achieved through a shift in equilibrium in areasonably controlled way, generally for a short period of time, togenerate sufficient fusing to obtain desired electrical conductivitywhile maintaining desired optical properties. In general, the fusingprocess can be performed with controlled exposure to a fusing vaporand/or through the use of a fusing agent in solution. Under appropriateconditions, the fused metal conductive network is a unitary structure.

In some embodiments, initiation of the fusing process can be controlledthrough a partial drying of the solutions to increase concentrations ofthe components, and quenching of the fusing process can be accomplished,for example, through rinsing or more completing drying of the metallayer. In some embodiments, subsequent processing with the deposition ofanother ink provides for the fusing of the metal nanowires into a metalnanostructured network, which is electrically conducting. The fusingagent can be incorporated into a single ink along with the metalnanowires. The one ink solution can provide appropriate control of thefusing process. A one ink formulation for forming a unitary fused metalconductive network with the thin, uniform metal nanowires is describedin the Examples below.

Sparse metal conductive layers are generally formed on a selectedsubstrate surface. In some embodiments, the substrate is a transparentpolymer film. Processing can be adapted for patterning of the film.Suitable polymers for a substrate include, for example, polyethyleneterephthalate (PET), polyethylene naphthalate (PEN), polyacrylate,poly(methyl methacrylate), polyolefin, polyvinyl chloride,fluoropolymers, polyamide, polyimide, polysulfone, polysiloxane,polyetheretherketone, polynorbornene, polyester, polystyrene,polyurethane, polyvinyl alcohol, polyvinyl acetate,acrylonitrile-butadiene-styrene copolymer, cyclic olefin polymer, cyclicolefin copolymer (COC), cyclic olefin polymers (COP), polycarbonate,copolymers thereof or blend thereof or the like. Fluoropolymers include,for example, polyvinylfluoride, polyvinylidenefluoride,polytetrafluoroethylene, hexafluoropropylene, perfluoropropylvinylether,perfluoromethylvinylether, polychlorotrifluoroethylene, and the like.Polymer films for some embodiments can have a thickness from about 5microns to about 5 mm, in further embodiments, from about 10 microns toabout 2 mm and in additional embodiment from about 15 microns to about 1mm. A person of ordinary skill in the art will recognize that additionalranges of thicknesses within the explicit ranges above are contemplatedand are within the present disclosure. Substrates can comprise aplurality of layers distinguished by composition and/or otherproperties. More specific ranges of materials suitable for substratesfor transparent conductive films are presented below, and the generalsubstrate ranges would include these specific materials and properties.

The films can be dried, for example, with a heat gun, an oven, a thermallamp or the like, although the films that can be air dried can bedesired in some embodiments. In some embodiments, the films can beheated to temperatures from about 50° C. to about 150° C. during drying.After drying, the films can be washed one or more times, for example,with an alcohol or other solvent or solvent blend, such as ethanol orisopropyl alcohol, to remove excess solids to lower haze. Patterning canbe achieved in several convenient ways. For example, printing of themetal nanowires can directly result in patterning. Additionally oralternatively, lithographic techniques and/or ablation methods can beused to remove or appropriately damage portions of the metal nanowires,prior to or after fusing, to form a pattern. One or more overcoat layerscan be applied over the sparse metal conductive layer, as describedabove.

For use of the transparent conductive films formed by the sparse metalconductive layers on the substrate, the films are generally integratedinto additional structures. For handling of the transparent conductivefilms, a protective polymer overcoat is generally placed over theconductive layer. Also, with respect to processing, the polymerovercoats can be applied using solution coating techniques, or otherprocessing approaches such as extrusion, lamination, calendering, meltcoating techniques or the like. If there is a plurality of polymerovercoats, they may or may not be applied using similar approaches. Forsolution processed overcoats, the various coating approaches describedabove, can be equally applied to these layers. However, the solutionprocessing of a polymer overcoat can be directed to solvents that arenot necessarily compatible with forming good dispersions of metalnanowires.

In general, the polymer overcoats can have average thicknesses fromabout 10 nanometers (nm) to about 12 microns, in further embodiments,from about 15 nm to about 10 microns and in additional embodiments fromabout 20 nm to about 8 microns. In some embodiments, it may be possibleto select an overcoat by choice of the refractive index and thicknesssuch that after application of the overcoat such that the haze issignificantly reduced without significantly degrading other properties.Also, the thickness and compositions of the overcoat can be selectedsuch that the sheet resistance measurement through the overcoat is notsignificantly altered relative to the measurement without the overcoat.A person of ordinary skill in the art will recognize that additionalranges of overcoat thicknesses within the explicit ranges above arecontemplated and are within the present disclosure.

Suitable commercial coating compositions for overcoats include, forexample, coating solutions from Dexerials Corporation (Japan), POSS®Coatings from Hybrid Plastics, Inc. (Mississippi, USA), silica filledsiloxane coatings from California Hardcoating Company (California, USA),CrystalCoat UV-curable coatings from SDC Technologies, Inc. (California,USA). The polymer concentrations and correspondingly the concentrationsof other non-volatile agents can be selected to achieve desired rheologyof the coating solution, such as an appropriate viscosity for theselected coating process. Solvent can be added or removed to adjusttotal solid concentrations. Relative amounts of solids can be selectedto adjust the composition of the finished coating composition, and thetotal amounts of solids can be adjusted to achieve a desired thicknessof the dried coating. Generally, the coating solution can have a polymerconcentration from about 0.025 wt % to about 50 wt %, in furtherembodiments from about 0.05 wt % to about 25 wt % and in additionalembodiments from about 0.075 wt % to about 20 wt %. A person of ordinaryskill in the art will recognize that additional ranges of polymerconcentrations within the specific ranges above are contemplated and arewithin the present disclosure. Polymers or composites thereof withdifferent refractive indices can be selected as overcoat materials fordesired optical transparency or other properties such as anti-reflectionand anti-glare.

The transparent coatings can further comprise optional propertymodifiers, such as crosslinking agents, wetting agents, viscositymodifiers, property modifying nanoparticles, and/or stabilizers, such asantioxidants and/or UV stabilizers, for transparent conductive films.The inclusion of property modifying nanoparticles, such as nanodiamonds,is described in published U.S. patent application 2016/0096967 to Virkaret al., entitled “Property Enhancing Fillers for Transparent Coatingsand Transparent Conductive Films,” incorporated herein by reference. Theincorporation of stabilizers into overcoats is described in copendingU.S. patent application Ser. No. 15/730,053 to Yang et al., entitled“Stabilized Sparse Metal Conductive Films and Solutions for Delivery ofStabilizing Compounds,” incorporated herein by reference.

For the deposition of the coating precursor solutions, any reasonabledeposition approach can be used, such as dip coating, spray coating,knife edge coating, bar coating, Meyer-rod coating, slot-die coating,gravure printing, spin coating or the like. The deposition approachdirects the amount of liquid deposited, and the concentration of thesolution can be adjusted to provide the desired thickness of productcoating on the surface. After forming the coating with the dispersion,the coating can be dried to remove the liquid and crosslinkedappropriately.

For the formation of products, optically clear adhesive layers andthicker protective films covering the sparse metal conductive layer,optionally with an overcoat as described above, can be formed with holesor the like in appropriate locations to provide for electricalconnections to the conductive layer. In general, various polymer filmprocessing techniques and equipment can be used to the processing ofthese polymer sheets, and such equipment and techniques are welldeveloped in the art, and future developed processing techniques andequipment can be correspondingly adapted for the materials herein. Theforming of these structures with optically clear adhesive layers and thelike are described further in published U.S. patent application2016/0122562A to Yang et al., entitled “Stabilized TransparentConductive Elements Based on Sparse Metal Conductive Layers,”incorporated herein by reference.

Transparent Conductive Films—CEI Color Properties

In the context of sparse metal conductive layers, the films can becharacterized by their color properties in addition to electricalconduction, transmittance and haze, which are discussed in the followingsection. Color parameters have been defined to correspond to colorperception. Measurements can be made to evaluate the color changesattributed to the transparent conductive film. In particular for silverbased sparse metal conductive layers, some yellowing can be observed,which generally is targeted to be kept at a low value, and diffusereflectance intensity (L*) is also desirable to be kept at a low valuefor low perceptional visibility. The thin, uniform nanowires describedherein can achieve excellent low color contributions that furthercontribute to their desirable nature for transparent conductive films.

Color spaces can be defined to relate spectral wavelengths to humanperception of color. CIELAB is a color space determined by theInternational Commission on Illumination (CIE). The CIELAB color spaceuses a three-dimensional set of coordinates, L*, a* and b*, where L*relates to the lightness of the color, a* relates to the position of thecolor between red and green, and b* relates to the position of the colorbetween yellow and blue. The “*” values represent normalized valuesrelative to a standard white point. The a* and b* CIELAB parameters canbe determined using commercial software from measurements made in aspectrophotometer based on transmission through a transparent conductivefilm. While the LAB parameters can be evaluated in either a transmissionor reflection configuration, herein a* and b* are evaluated in atransmission configuration, associated with the transparent conductivefilm, and L* is evaluated in a reflection configuration with a colormeter/spectrophotometer, measured using a stacked structure as furtherdescribed below. The L* measurements reported herein are diffusereflective values, which exclude the speculative reflections. Thespectrophotometer used to make the measurement has an integration spherethat can perform the measurement over the integration sphere excludingthe specular reflection.

Films incorporating a sparse metal conductive network can be found tohave a yellowish tint, and the lowering of b* can result in a moreneutral appearance of the film. The thin, uniform silver nanowiresdescribed herein have been found to form highly transparent films withlow sheet resistance with relatively low values of b*. Also, it has beenfound that nanoscale colorants can be introduced into the structures tolower the overall b* values of the structures. The use of nanoscalecolorants is described in published U.S. patent application 2016/0108256to Yang et al., entitled “Transparent Films with Control of Light HueUsing Nanoscale Colorants,” incorporated herein by reference.

For some applications, the value of reflective L* is also significant.To measure L*, it is often desirable to make the measurement against ablack background in a reflection orientation. L* relates to thelightness of the color, and a value of L* near zero, corresponding toblack, can be desirable based on a reflection measurement on a stackedstructure with black background. While the color may be close to black,the transmission through the conductive layer is high, so thetransparent conductive film just contributes a dark hue to the extentthat the color is changed. For certain applications, it can be desirablefor the transparent conductive film to contribute little to thereflection of light relative to the black background. As reported in theExamples, values of ΔL* are reported, which are equal toL*(TCF)−L*(base), where the L*(TCF) are values for structures with atransparent conductive film mounted on the substrate with a blackbackground, and L*(base) is that for the corresponding structure withoutthe transparent conductive layer. The measurements are performed indiffusive reflection (or SCE, Specular Contribution Excluded) mode usinga Konica-Minolta Spectrophotometer CM-3700A, which is capable oftransmission or reflection measurements and has a built in integrationsphere, equipped with SpectraMagic NX software, that can provide CIELABvalues.

In some embodiments, the transparent conductive films of no more thanabout 100 ohms/sq in sheet resistance can result in a value of ΔL* of nomore than about 2.0, in further embodiments no more than about 1.75, andin additional embodiments no more than about 1.5 when measured in areflection configuration against a black background. Also, it can bedesirable for the absolute value of transmission b* for the transparentconductive film to be no more than 1.0, in further embodiments no morethan 0.9 and in additional embodiments no more than a value of 0.75. Aperson of ordinary skill in the art will recognize that additionalranges of optical parameters and corresponding sheet resistances withinthe explicit ranges above are contemplated and are within the presentdisclosure. Values of b* and a* can be evaluated using the equations inthe standard CIELAB94 and/or CIE DE2000, Center International Commissionon Illumination (Commission Internationale de L'Eclairage), seeColorimetry, 3rd Edition, CIE, 2004, incorporated herein by reference.These calculations can be performed using commercial spectrophotometersand software, such as Konica Minolta Spectrophotometer CM-3700A withSpectraMagic™ NX software.

Transparent Conductive Films—Electrical and Optical Properties

Transparent conductive films incorporating sparse metal conductivelayers, e.g., fused metal nanostructured networks, can provide lowelectrical resistance while providing good optical properties. Thus, thefilms can be useful as transparent conductive electrodes or the like.The transparent conductive electrodes can be suitable for a range ofapplications such as electrodes along light receiving surfaces of solarcells. For displays and in particular for touch screens, the films canbe patterned to provide electrically conductive patterns formed by thefilm. The substrate with the transparent conductive film, generally hasgood optical properties at the respective portions of the pattern.

Electrical resistance of thin films can be expressed as a sheetresistance, which is reported in units of ohms per square (Ω/□ orohms/sq) to distinguish the values from bulk electrical resistancevalues according to parameters related to the measurement process. Sheetresistance of films is generally measured using a four point probemeasurement or another suitable process. In some embodiments, the fusedmetal nanowire networks can have a sheet resistance of no more thanabout 200 ohms/sq, in further embodiments no more than about 150ohms/sq, in additional embodiments no more than about 100 ohms/sq and inother embodiments no more than about 75 ohms/sq. A person of ordinaryskill in the art will recognize that additional ranges of sheetresistance within the explicit ranges above are contemplated and arewithin the present disclosure. In the Examples, transparent conductivefilms are formed with target sheet resistances of 70 ohms/sq or 50ohms/sq. In general, sheet resistance can be reduced by increasing theloading of nanowires, but an increased loading may or may not bedesirable from other perspectives.

Depending on the particular application, commercial specifications forsheet resistances for use in a device may not be necessarily directed tolower values of sheet resistance such as when additional cost may beinvolved, and current commercially relevant values may be for example,270 ohms/sq, versus 150 ohms/sq, versus 100 ohms/sq, versus 50 ohms/sq,versus 40 ohms/sq, versus 30 ohms/sq or less as target values fordifferent quality and/or size touch screens, and each of these valuesdefines a range between the specific values as end points of the range,such as 270 ohms/sq to 150 ohms/sq, 270 ohms/sq to 100 ohms/sq, 150ohms/sq to 100 ohms/sq and the like with 15 particular ranges beingdefined. Thus, lower cost films may be suitable for certain applicationsin exchange for modestly higher sheet resistance values.

For applications as transparent conductive films, it is desirable forthe fused metal nanowire networks or other sparse metal conductive layerto maintain good optical transparency. In principle, opticaltransparency is inversely related to the metal loading with higherloadings leading to a reduction in transparency, although processing ofthe network can also significantly affect the transparency. Also,polymer binders and other additives can be selected to maintain goodoptical transparency. Transparency can be reported in terms of adimensionless quantity, transmittance. Transmittance is the ratio of thetransmitted light intensity (I) to the incident light intensity (I_(o)).Transmittance is reported in the Examples below for the transparentconductive layer on a transparent substrate. The optical transparency ofthe transparent conductive layer can be evaluated relative to thetransmitted light through the substrate supporting the transparentconductive film. For example, the transparency of the conductive layerdescribed herein can be measured by using a UV-Visible spectrophotometerand measuring the total transmission through the conductive layer andsupport substrate. Transmittance values are reported for the transparentconductive layers on the transparent film substrates. The transmittancethrough the conductive layer (T_(layer)) can be estimated by dividingthe total transmittance (T) measured by the transmittance through thesupport substrate (T_(sub)). (T=I/I_(o) andT/T_(sub)=(I/I_(o))/(I_(sub)/I_(o))=I/I_(sub)=T_(layer)). Thus, thereported total transmittance can be corrected to remove thetransmittance through the substrate to obtain transmittance of aconductive layer, overcoat or other component alone.

While it is generally desirable to have good optical transparency acrossthe visible spectrum, for convenience, optical transmission can bereported at 550 nm wavelength of light. Alternatively or additionally,transmission can be reported as total transmittance from 400 nm to 700nm wavelength of light, and such results are reported in the Examplesbelow. In general, for the fused metal nanowire films, the measurementsof 550 nm transmittance and total transmittance from 400 nm to 700 nm(or just “total transmittance” for convenience) are not qualitativelydifferent. In some embodiments, the film formed by the fused network hasa total transmittance (TT %) of at least 80%, in further embodiments atleast about 85%, in additional embodiments, at least about 90%, in otherembodiments at least about 94% and in some embodiments from about 95% toabout 99%. Transmittance of the films can be evaluated using thestandard ASTM D1003 (“Standard Test Method for Haze and LuminousTransmittance of Transparent Plastics”), incorporated herein byreference. A person or ordinary skill in the art will recognize thatadditional ranges of transmittance within the explicit ranges above arecontemplated and are within the present disclosure.

The fused metal networks can also have low haze along with hightransmission of visible light while having desirably low sheetresistance. Haze can be measured using a haze meter based on ASTM D1003referenced above, and the haze contribution of the substrate can beremoved to provide haze values of the transparent conductive film. Hazeas measured based on ASTM D1003 is a transmission based haze, andseparate measurements can be made for reflection based haze. Unlessexplicitly indicated otherwise, haze refers to transmission based haze.In some embodiments, the transparent conductive film can have a hazevalue of no more than about 0.6%, in further embodiments no more thanabout 0.5%, in additional embodiments no more than about 0.4% and inother embodiments from about 0.35% to about 0.15%. It has been foundthat selected transparent overcoats can significantly reduce the hazefrom the structure with the transparent conductive layer, and the valuesof the structure with the overcoat can be considered with respect tothese ranges of haze values. Polymer overcoats are discussed above. Aperson of ordinary skill in the art will recognize that additionalranges of haze within the explicit ranges above are contemplated and arewithin the present disclosure.

In the Examples below, transparent conductive films are formed with twotarget sheet resistance values, 70 ohms/sq or 50 ohms/sq, although theactual sheet resistance values are measured. To obtain lower values ofsheet resistance, a higher metal loading on the surface can be used,which can correspondingly result in a slight reduction of transmittancealong with an increase in haze and b*.

As described in the Examples, with appropriately selected silvernanowires very low values of haze and sheet resistance have beensimultaneously achieved. A polymer overcoat can further reduce the haze.The loading can be adjusted to balance the sheet resistance and the hazevalues with very low haze values possible with still good sheetresistance values. Specifically, haze values of no more than 0.8%, andin further embodiments from about 0.15% to about 0.7%, can be achievedwith values of sheet resistance of at least about 60 ohms/sq. Also, hazevalues of 0.3% to about 0.8%, and in some embodiments from about 0.35%to about 0.7%, can be achieved with sheet resistance values of fromabout 30 ohms/sq to about 60 ohms/sq. All of these films maintained goodoptical transparency. A person of ordinary skill in the art willrecognize that additional ranges of haze within the explicit rangesabove are contemplated and are within the present disclosure.

With respect to the corresponding properties of the multilayered films,the additional components are generally selected to have a small effecton the optical properties, and various coatings and substrates arecommercially available for use in transparent elements. Suitable opticalcoatings, substrates and associated materials are summarized above. Someof the structural material can be electrically insulating, and ifthicker insulating layers are used, the film can be patterned to providelocations where gaps or voids through the insulating layers can provideaccess and electrical contact to the otherwise embedded electricallyconductive element.

For the production of sensors or the like, patterning is generallyperformed as outlined above to generate electrically conductive regionsand electrically insulating regions, which are both transparent.Transparent conducting films made from inks comprised of the thinuniform nanowires can also be coated onto one or more sides of asubstrate and can be used to fabricate touch sensors. While touchsensors can be composed of two distinct layers of transparent conductivefilm separated by an optically clear adhesive, it is also possible tocreate a sensor by depositing (coating) and then patterning the sparsemetal layer onto one substrate. A single coating of the sparse metallayer on one side of the substrate can be patterned and processed todefine both the sense and ground (or “X”- and “Y”-) on a single layer.Alternatively, the substrate can be coated on both sides of the samesubstrate, such as with sequential deposition and processing on eachside. After patterning to and further processing to incorporate thetransparent conductive film(s) into the structure, ultrathin sensors canbe fabricated. One major advantage of the transparent conductors madefrom sparse metal layers compared to indium tin oxide (or other dopedmetal oxides) is superior flexibility. By creating the entire sensor ona single substrate, this advantage can be further exploited. The generalpatterning of a substrate with transparent conductors on both surfacesis described in published U.S. patent application 2015/0116255 to Zhonget al., entitled “Double Sided Touch Sensor on Transparent Substrate,”incorporated herein by reference. The processing of silver nanowirebased transparent conductive films on opposite sides of a substrate isdescribed in published US patent application 2014/0202742 to Jones etal., entitled “Two-Sided Laser Patterning on Thin Film Substrates,”incorporated herein by reference.

EXAMPLES Example 1—Silver Nanowire Synthesis with Salt CatalystsIncluding KBr

This example described the synthesis of very thin and highly uniformsilver nanowires using improved catalysts combination and processing.

A first synthesis reaction was performed in a closed two liter reactorand four batches were combined from each embodiment for analysis.Initially, about 1.67 L of ethylene glycol was placed in the reactor atroom temperature, and heating was started. Before heating was started,polyvinylpyrrolidone (K30, BASF) was added to the EG under continuousstirring, and heating was continued. Once the reactor almost reached thetarget peak temperature, heating was stopped, and ethylene glycolsolutions of the catalysts (KBr and 1-octyl-3-methylimidazolium chloride(OMMCl)) were added to the reactor under continued mixing. Afterappropriate mixing, a solution of silver nitrate in ethylene glycol wasadded. Based on the added quantities to the reactor, the concentrationsin the reactor are PVP 0.1 wt % to 10 wt %, KBr 0.00002M to 0.002M,OMMCl 0.00002M to 0.002M, AgNO₃ 0.0025M to 0.25M. Of course, once thereactants are added and various reactions take place, such as AgClprecipitation and Ag reduction, the concentrations in solution of thevarious species evolve, such that reference to the quantities as addedprovides a useful reference point. After silver nitrate is mixed, anadditional amount of ethylene glycol was added to reach the desiredvolume, and stirring was continued until the end of the reaction roughlyan hour after the last addition to the reactor.

Following completion of the synthesis, the silver nanowires werepurified using repeated acetone precipitation, centrifugation andre-dispersion in water. The purified silver nanowires were removed fromdispersion and dried to evaluate yield. Yield is based on the quantityof silver in the purified silver nanowires compared to the total amountof silver added in the form of silver nitrate. For this experiment, theyield was generally about 12-15%.

The collected silver nanowires were characterized by transmissionelectron microscopy (TEM). A representative TEM micrograph is shown inFIG. 1 . The wire diameters were measured and averaged for 108 wires. Ahistogram of the silver nanowire diameters is shown in FIG. 2 . The meandiameter was 17.3 nm, with a standard deviation of 1.9 nm, 69% of thenanowires having a diameter of 18 nm or less and 1.8% of the nanowireshaving a diameter of 15 nm or less. The smallest measured diameter was14.2 nm.

A dispersion of the silver nanowires was formed in DMSO, and theabsorption spectrum was taken. A representative UV-visible absorptionspectrum is shown in FIG. 3 along with an absorption spectrum of arepresentative control synthesized without the imidazolium chloridesalt. The same control absorption spectrum is shown in all of thefollowing absorption spectrum plots. The absorption maximum was at 375.5nm compared with the control absorption maximum of 376.6 nm, and thenormalized absorption at 410 nm was 0.208 compared with the controlnormalized absorption at 410 nm of 0.246. The reduced absorption at 410nm is consistent with thinner and less hazy nanowires.

A second synthesis reaction was performed with PVP K90 (BASF)substituted for the PVP K30. The remaining aspects of the synthesis weresubstantially the same as the first synthesis above. The yield for thisreaction was low. A representative electron micrograph for this sampleis shown in FIG. 4 . The wire diameters were measured and averaged for52 wires. A histogram of the silver nanowire diameters is shown in FIG.5 . The mean diameter was 16.2 nm, with a standard deviation of 2.1 nm,82.7% of the nanowires having a diameter of 18 nm or less and 29% of thenanowires having a diameter of 15 nm or less. The smallest measureddiameter was 12.7 nm. A third synthesis reaction was performed with PVPK85 (commercial more uniform polymer) substituted for 3% of original PVPK30. The remaining aspects of the synthesis were substantially the sameas for the first synthesis above. The yield for this reaction was about12%.

A representative TEM micrograph is shown in FIG. 6 . The wire diameterswere measured and averaged for 128 wires. A histogram of the silvernanowire diameters is shown in FIG. 7 . The mean diameter was 16.9 nm,with a standard deviation of 1.7 nm, 74.3% of the nanowires having adiameter of 18 nm or less and 8.6% of the nanowires having a diameter of15 nm or less. The smallest measured diameter was 14.2 nm. The resultsobtained with PVP (high molecular weight) K85 was intermediate betweenthe silver nanowire properties from the synthesis with PVP K30 and withPVP K90, except that the standard deviation of the nanowire diameter wassmaller for silver nanowires synthesized with PVP K85 (high molecularweight) than for either of the other synthesis results. A representativeabsorption spectrum is shown in FIG. 8 . The absorption maximum was at372.1 nm and the normalized absorption at 410 nm was 0.166. The narrowpeak width and the small absorption at 410 nm is consistent with thehigh degree of diameter uniformity and thin nanowires.

Example 2—Silver Nanowire Synthesis with Alternative Bromide SaltCatalysts

This example is directed to silver nanowire synthesis using bromide saltcatalysts with polyatomic cations.

A first set of experiments was performed with hexyltrimethyl ammoniumbromide (C₆H₁₃(CH₃)₃NBr) along with the 1-octyl-3-methylimidazoliumchloride catalyst. The remaining aspects of the synthesis were performedas described in Example 1. A representative TEM micrograph is shown inFIG. 9 . The wire diameters were measured and averaged for 123 wires. Ahistogram of the silver nanowire diameters is shown in FIG. 10 . Themean diameter was 17.3 nm, with a standard deviation of 2.1 nm, 81.3% ofthe nanowires having a diameter of 18 nm or less and 5.7% of thenanowires having a diameter of 15 nm or less. The smallest measureddiameter was 14.4 nm. A dispersion of the silver nanowires was formed inDMSO, and the absorption spectrum was taken. A representative absorptionspectrum is shown in FIG. 11 . The absorption maximum was at 372.1 nmand the normalized absorption at 410 nm was 0.176. The small absorptionat 410 nm is consistent with the high degree of diameter uniformity.

Another sample was synthesized using 1-butyl-3-methylimidazolium bromidesubstituted for KBr of Example 1. The remaining aspects of the synthesiswere performed as described in Example 1. A representative TEMmicrograph is shown in FIG. 12 . The wire diameters were measured andaveraged for 134 wires. A histogram of the silver nanowire diameters isshown in FIG. 13 . The mean diameter was 17.4 nm, with a standarddeviation of 1.8 nm. The smallest measured diameter was 13.7 nm, whilethe largest measured diameter was 23.6 nm.

Example 3—Silver Nanowires Synthesized with Imidazole

This example demonstrates the properties of silver nanowires synthesizedusing an imidazole organic catalyst.

Two samples were formed using imidazole additive along with ammoniumchloride and potassium bromide salt catalysts along with a controlsample that just used ammonium chloride and potassium bromide withoutany imidazole additive. The imidazole additive was added atconcentrations from about 0.0001M to about 0.05M. Except for theaddition of the imidazole, the synthesis reactions were essentially asperformed for Example 1. Two different concentrations of imidazole wereused in the respective samples, with three times the imidazole used inthe second sample relative to the first sample.

A representative TEM micrograph is shown for the control nanowire samplein FIG. 14 . The wire diameters were measured and averaged for 100wires. A histogram of the silver nanowire diameters is shown in FIG. 15. The mean diameter was 21.5 nm, with a standard deviation of 2.4 nm.The smallest measured diameter was 17.2 nm. A dispersion of the silvernanowires was formed in DMSO, and the absorption spectrum was taken. Arepresentative absorption spectrum is shown in combination with a samplewith imidazole, as explained further below. The absorption maximum wasat 377.6 nm and the normalized absorption at 410 nm was 0.243.

A representative TEM micrograph of silver nanowires synthesized at thefirst imidazole concentration is shown in FIG. 16 . The wire diameterswere measured and averaged for 108 wires. A histogram of the silvernanowire diameters is shown in FIG. 17 . The mean diameter was 19.6 nm,with a standard deviation of 1.8 nm, 20% of the nanowires having adiameter of 18 nm or less and 0% of the nanowires having a diameter of15 nm or less. The smallest measured diameter was 16.8 nm. A dispersionof the silver nanowires was formed in DMSO, and the absorption spectrumwas taken. A representative absorption spectrum is shown in FIG. 18along with the spectrum for the control with no imidazole. Theabsorption maximum was at 376.0 nm and the normalized absorption at 410nm was 0.214. The nanowires synthesize with the imidazole had a reducednanowire diameter and more uniformity as expressed by the standarddeviation, UV-visible peak width, the wavelength of absorption maximum,and relative absorption at 410 nm.

A representative TEM micrograph of silver nanowires synthesized at thesecond imidazole concentration is shown in FIG. 19 . The wire diameterswere measured and averaged for 105 wires. A histogram of the silvernanowire diameters is shown in FIG. 20 . The mean diameter was 18.9 nm,with a standard deviation of 2.0 nm, 38% of the nanowires having adiameter of 18 nm or less and 0% of the nanowires having a diameter of15 nm or less. The smallest measured diameter was 15.5 nm. A dispersionof the silver nanowires was formed in DMSO, and the absorption spectrumwas taken. A representative absorption spectrum is shown in FIG. 21along with the spectrum for the control with no imidazole. Theabsorption maximum was at 376 nm and the normalized absorption at 410 nmwas 0.206. The nanowires synthesize with the greater imidazoleconcentration had a further reduced nanowire diameter, although theuniformity as expressed by the standard deviation did not furtherimprove.

Two additional silver nanowire syntheses were performed at two imidazoleconcentrations with 1-octyl-3-methyl imidazolium chloride and potassiumbromide salts. The second imidazole concentration was double the firstimidazole concentration. A representative TEM micrograph of the silvernanowires synthesized using the first imidazole concentration is shownin FIG. 22 . The wire diameters were measured and averaged for 109wires. A histogram of the silver nanowire diameters is shown in FIG. 23. The mean diameter was 15.8 nm, with a standard deviation of 1.5 nm,90% of the nanowires having a diameter of 18 nm or less and 32% of thenanowires having a diameter of 15 nm or less. The smallest measureddiameter was 13.3 nm. A dispersion of the silver nanowires was formed inDMSO, and the absorption spectrum was taken. A representative absorptionspectrum is shown in FIG. 24 along with the spectrum for the controlwith no imidazole. The absorption maximum was at 373.5 nm compared witha control absorption maximum of 376.1 nm and the normalized absorptionat 410 nm was 0.188 compared with a control 410 nm normalized absorptionof 0.216. The nanowires synthesized with the imidazole had a very smalland uniform nanowire diameter, based on standard deviation, and theabsorption spectrum.

A representative TEM micrograph of the silver nanowires synthesizedusing the second imidazole concentration (along with the1-octyl-3-methyl imidazolium chloride and potassium bromide) is shown inFIG. 25 . The wire diameters were measured and averaged for 102 wires. Ahistogram of the silver nanowire diameters is shown in FIG. 26 . Themean diameter was 17.7 nm, with a standard deviation of 2.8 nm, 70.6% ofthe nanowires having a diameter of 18 nm or less and 12.7% of thenanowires having a diameter of 15 nm or less. The smallest measureddiameter was 13.7 nm. A dispersion of the silver nanowires was formed inDMSO, and the absorption spectrum was taken. A representative absorptionspectrum is shown in FIG. 27 along with the spectrum for the controlwith no imidazole. The absorption maximum was at 374.4 nm compared witha control absorption maximum of 376.1 nm and the normalized absorptionat 410 nm was 0.178 compared with a control 410 nm normalized absorptionof 0.216. The greater imidazole concentration resulted in thicker andless uniform silver nanowires relative to the sample described in thepreceding paragraph with a lesser imidazole concentration.

Example 4—Silver Nanowires Synthesized with Pyrazole

This example demonstrates the properties of silver nanowires synthesizedusing a pyrazole organic catalyst.

Two silver nanowire samples were formed at a scale for a 2 liter reactorusing pyrazole additive along with 1-octyl-3-methyl imidazolium chlorideand potassium bromide salts. The pyrazole additive was added atconcentrations from about 0.0001M to about 0.05M. Except for theaddition of the pyrazole, the synthesis reactions were essentially asperformed for Example 2. The synthesis was repeated at the sameconcentration of pyrazole for two samples.

A representative TEM micrograph of the silver nanowires synthesizedusing the first pyrazole sample is shown in FIG. 28 . The wire diameterswere measured and averaged for 103 wires. A histogram of the silvernanowire diameters is shown in FIG. 29 . The mean diameter was 14.9 nm,with a standard deviation of 1.65 nm, 94% of the nanowires having adiameter of 18 nm or less and 62% of the nanowires having a diameter of15 nm or less. The smallest measured diameter was 12.5 nm. A dispersionof the silver nanowires was formed in DMSO, and the absorption spectrumwas taken. A representative absorption spectrum is shown in FIG. 30along with the spectrum for the control with no pyrazole. The absorptionmaximum was at 368 nm compared with a control absorption maximum of376.1 nm and the normalized absorption at 410 nm was 0.154 compared witha control 410 nm normalized absorption of 0.216. The nanowiressynthesized with the pyrazole had a very small and uniform nanowirediameter with an even smaller average diameter than synthesized withimidazole and a similar standard deviation.

A representative TEM micrograph of the silver nanowires synthesized inthe second run of the selected pyrazole concentration is shown in FIG.31 . The wire diameters were measured and averaged for 90 wires. Ahistogram of the silver nanowire diameters is shown in FIG. 32 . Themean diameter was 15.25 nm, with a standard deviation of 1.73 nm, 95% ofthe nanowires having a diameter of 18 nm or less and 54% of thenanowires having a diameter of 15 nm or less. The smallest measureddiameter was 12.3 nm. A dispersion of the silver nanowires was formed inDMSO, and the absorption spectrum was taken. A representative absorptionspectrum is shown in FIG. 33 along with the spectrum for the controlwith no pyrazole. The absorption maximum was at 370 nm compared with acontrol absorption maximum of 376.1 nm and the normalized absorption at410 nm was 0.154 compared with a control 410 nm normalized absorption of0.216. The nanowires synthesized in the second run with pyrazole had asimilar very small and uniform nanowire diameter although with aslightly larger average diameter and standard deviation relative to thesilver nanowires synthesized at the first pyrazole run.

A further silver nanowire sample was formed at a scale for a 10 literreactor using pyrazole additive at the concentration of the samplesabove along with 1-octyl-3-methyl imidazolium chloride and potassiumbromide salts. A representative TEM micrograph of the silver nanowiressynthesized using the second pyrazole concentration is shown in FIG. 34. The wire diameters were measured and averaged for 94 wires. Ahistogram of the silver nanowire diameters is shown in FIG. 35 . Themean diameter was 15.29 nm, with a standard deviation of 1.21 nm, 99% ofthe nanowires having a diameter of 18 nm or less and 39% of thenanowires having a diameter of 15 nm or less. The smallest measureddiameter was 12.9 nm. A dispersion of the silver nanowires was formed inDMSO, and the absorption spectrum was taken. A representative absorptionspectrum is shown in FIG. 36 along with the spectrum for the controlwith no pyrazole. The absorption maximum was at 371 nm compared with acontrol absorption maximum of 376.1 nm and the normalized absorption at410 nm was 0.158 compared with a control 410 nm normalized absorption of0.216. Absorption spectra were also taken for dispersions in isopropylalcohol, and for all three samples, the absorption maximum was at 358.5nm for the first sample and at 359 nm for the other two samples. Thenanowires synthesized at the ten liter scale generally had roughlyequivalent properties to the silver nanowires produced in smallerbatches within presently observed batch to batch variation.

Example 5—Addition of Water During Synthesis

The Example demonstrates that the addition of a small amount of waterduring the synthesis of the nanowires can result in the reduction of thenanowire diameter while maintaining uniform nanowires.

A sample of silver nanowires was synthesized as described in Example 1with OMMCl and KBr catalysts, but with the addition of 0.3 weightpercent water that was added to the solvent prior to the addition ofsilver nitrate. A representative TEM micrograph of the silver nanowiressynthesized using the small amount of water in the reaction is shown inFIG. 37 . The wire diameters were measured and averaged for 127nanowires. A histogram of the silver nanowire diameters is shown in FIG.38 . The mean diameter was 16.5 nm, with a standard deviation of 1.69 nmand 72% of the nanowires having a diameter of 17 nm or less. Thesmallest measured diameter was 14.3 nm. A dispersion of the silvernanowires was formed in DMSO, and the absorption spectrum was taken. Arepresentative absorption spectrum is shown in FIG. 39 along with thespectrum for a representative control without water addition. Theabsorption maximum was at 373 nm compared with a control absorptionmaximum of 376.6 nm and the normalized absorption at 410 nm was 0.182compared with a control 410 nm normalized absorption of 0.246. Theinclusion of the water reduced the average diameter by 0.8 nm andresulted in a greater uniformity of the nanowires.

Example 6—Transparent Conductive Films

This example establishes the improved properties of transparentconductive films formed from the thinner and uniform silver nanowiressynthesized as described herein.

The optical properties of the transparent conductive films were examinedfor selected sets of the thinner and uniform silver nanowiressynthesized as described in the previous examples. Measurements areperformed on two structures along with control measurements of L* usinga reference structure with no transparent conductive film. For both ofthe conductive structures, the transparent conductive film is formedusing a silver nanowire ink essentially as described in Example 5 of the'968 patent, cited above. The film is deposited using slot coating byhand onto the substrate. The structures with the silver nanowire inkswere dried in an oven at 100° C. for up to 10 minutes to dry the inks.The drying process induces chemical fusing as the solvent is removedconcentrating the constituents, in which further reaction is ended bythe continued drying. The amount of silver deposited is selected totarget a film with roughly either 50 ohms/sq or 70 ohms/sq sheetresistance, as noted in the specific samples below. After drying theconductive film, a commercial overcoat is slot coated by hand over thedried transparent conductive film. The overcoat is dried and crosslinkedwith a UV lamp to form an overcoat with a thickness of about 85 nm.

With respect to the structures for testing the film properties, thesestructures are shown schematically in FIGS. 40-42 . Referring to FIG. 40, a transparent structure 100 is shown with a transparent conductivelayer 102 (having a fused metal nanostructured network) on a substrate104 (50 micron thick cyclic olefin polymer (COP)) with a roughly 85 nmovercoat layer 106 formed with the commercial acrylate based coatingcomposition over transparent conductive layer 102. Referring to FIG. 41, a second structure 110 is shown that is suitable for reflectivemeasurements. A black acrylic board is used to provide a blackbackground for the measurements. In some sense, the black substrate isused to mimic a black cell phone screen. In FIG. 41 , an optically clearadhesive layer 112 with two adhesive surfaces is placed over overcoatlayer 106. The other surface of optically clear adhesive layer 112 isplaced on black acrylic board 114. An antireflective layer 116 is placedon the other surface of substrate 104. A reference structure 120 isshown in FIG. 42 . Referring to FIG. 42 , black acrylic board 114optically clear adhesive layer 112, substrate 104 and antireflectivelayer 116 are stacked without the transparent conductive layer 102 orthe overcoat layer 106.

This structure of FIG. 40 is used to measure sheet resistance,transmittance, b* and haze. The product of the sheet resistance and hazeis a useful reference point to adjust for variation in amounts ofdeposited nanowires, so this product is also tabulated below. The secondstructure is shown in FIG. 41 , and a reference structure without thetransparent conductive film is shown in FIG. 42 . The structures inFIGS. 41 and 42 are used to measure reflective L* in SCE mode. TheCIELAB measurements in SCE reflective mode for reference structure 120of FIG. 42 were L*=1.62, a*=0.19, and b*=−0.07.

The haze values of the film samples were measured using a BYK Haze-Gardplus 4725 Haze Meter. To adjust the haze measurements for the samplesbelow, a value of substrate haze can be subtracted for a control sample(FIG. 42 ) from the measurements with the transparent conductive layer(FIG. 41 ) to get approximate haze measurements for the transparentconductive films and overcoat separately. Values of TT % and haze weremeasured with the BYK Haze-Gard plus 4725 Haze Meter based on ASTM D1003 standard (“Standard Test Method for Haze and Luminous Transmittanceof Transparent Plastics”), incorporated herein by reference. The totaltransmission and haze values presented for the films include the 50micron thick COP substrate which has total transmission 92.6%, haze0.08%, and b* 0.09. CIELAB values of b* and a* were determined usingcommercial software from measurements made with a Konica MinoltaSpectrophotometer CM-3700A with SpectraMagic™ NX software.

Sheet resistance was measured with a 4-point probe method, a contactlessresistance meter or by measuring the resistance of the film by using asquare defined by two solid (non-transparent) lines of silver formedfrom silver paste. In some embodiments, to make sheet resistancemeasurements, a pair of parallel stripes of silver paste was sometimeused by painting the paste onto the surface of the samples to define asquare, or a rectangular shape, which were then annealed at roughly 120°C. for 20 minutes in order to cure and dry the silver paste. Alligatorclips were connected to the silver paste stripes, and the leads wereconnected to a commercial resistance measurement device.

The conduction and optical measurements were performed for controlnanowires and for nanowires with the reduced diameters and uniformity.Measurements were performed with two deposition amounts, one to target asheet resistance of about 50 ohms/sq and the second to target about 70ohms/sq, as well as measurements with and without the overcoat. Theresults are presented in Tables 1 (w/o overcoat) and 2 (with overcoat).The product of the sheet resistance and haze is significantly reducedfor the thinner, uniform nanowires.

TABLE 1 AgNW (Average Before OC Diameter) Ω/□ TT % H % b* Batch 1~15 nm45 91.3 0.49 0.90 Batch 1~15 nm 64 91.6 0.36 0.66 Standard~18 nm 48 91.40.54 0.76 Standard~18 nm 74 91.7 0.36 0.55

TABLE 2 AgNW (Average After OC Diameter) Ω/□ TT % H % b* Batch 1~15 nm46 91.5 0.36 0.83 Batch 1~15 nm 67 91.8 0.27 0.67 Standard~18 nm 50 91.50.40 0.76 Standard~18 nm 75 91.8 0.35 0.60

The conduction and optical properties using the three different thin,uniform nanowire batches with similar nanowire properties were alsodetermined. The results are shown in Table 3 (without overcoat) andTable 4 (with overcoat). In these results, as in the results presentedin Tables 1 and 2, the overcoat resulted in a significant decrease in H% and b* while resulting in little or no change in transmittance (TT %).

TABLE 3 Average AgNW Diameter, Before OC Batch nm (TEM) Ω/□ TT % H % b*Batch 2 ~14-15 46 91.4 0.51 1.09 Batch 3 ~14-15 39 91.3 0.56 1.09 Batch4 ~14-15 45 91.3 0.50 0.99

TABLE 4 Average AgNW Diameter, After OC Batch nm (TEM) Ω/□ TT % H % b*Batch 2 ~14-15 45 91.3 0.37 0.82 Batch 3 ~14-15 38 91.3 0.37 0.92 Batch4 ~14-15 45 91.5 0.33 0.84

L* was measured in reflection configuration for the samples used togenerate Tables 1 and 2. L* was measured for the control structure inFIG. 42 and was found to be 1.62. For the samples with the transparentconductive film, the value of ΔL* is reported, which isL*sample-L*control, since this value relates to the properties of thetransparent conductive film. The values are presented in Table 5. Thethinner nanowires exhibited a significantly smaller value of ΔL*.

TABLE 5 Stack Sample Ω/□ ΔL* Batch 1~15 nm 46 2.12 Batch 1~15 nm 67 1.49Standard~18 nm 50 2.25 Standard~18 nm 75 1.67

Seven batches of thinner, uniform silver nanowires were evaluated toexamine how the diameter distributions influenced the opticalproperties. The coatings were targeted to have a sheet resistance ofabout 70 ohms/sq, although the specific values measured are tabulated.The nanowire properties are presented in Table 6 and the opticalproperties for the nanowires are presented in Table 7.

TABLE 6 Avg. Std. NW Diameter Dev. Max Batch (nm) (nm) (nm) % < 18 nm %< 15 nm % < 13 nm 1 15.5 2.85 31.0 86.9 48.0 12.7  2 15.4 2.36 29.0 92.950.4 4.4 3 15.2 2.39 23.0 88.7 64.4 9.6 4 14.9 1.65 20.6 94.2 62.1 3.9 514.5 1.56 19.5 98.3 64.1 20.5  6 15.3 1.73 21.5 95.4 54.6 7.4 7 15.31.21 18.2 98.9 38.7 1.1

TABLE 7 NW Batch Ω/□ % H b* ΔL* 1 66 0.23 0.62 1.43 2 68 0.28 0.58 1.393 69 0.29 0.62 1.48 4 70 0.27 0.60 1.35 5 76 0.20 0.56 1.25 6 70 0.250.54 1.40 7 66 0.23 0.63 1.28

Based on the results in Tables 5 and 6, lower values of L* can beassociated with having fewer nanowires with diameters greater than 18nm. Thus, both the thinner average diameters in conjunction with a smallstandard deviation can be particularly significant for achieving a smallvalue of L*.

A structure was formed with fused metal nanostructured networks on theopposing surfaces of a COP polymer substrate. The structure is shown inFIG. 43 . The transparent conductive films and polymer overcoats wereformed sequentially, and each transparent conductive film was formedessentially as described above for the earlier results in this example.The resulting structure with two transparent conductive films each filmhad a sheet resistance of approximately 70 ohms/sq. The haze (H %) forthe final structure was 0.56.

The embodiments above are intended to be illustrative and not limiting.Additional embodiments are within the claims. In addition, although thepresent invention has been described with reference to particularembodiments, those skilled in the art will recognize that changes can bemade in form and detail without departing from the spirit and scope ofthe invention. Any incorporation by reference of documents above islimited such that no subject matter is incorporated that is contrary tothe explicit disclosure herein. To the extent that specific structures,compositions and/or processes are described herein with components,elements, ingredients or other partitions, it is to be understand thatthe disclosure herein covers the specific embodiments, embodimentscomprising the specific components, elements, ingredients, otherpartitions or combinations thereof as well as embodiments consistingessentially of such specific components, ingredients or other partitionsor combinations thereof that can include additional features that do notchange the fundamental nature of the subject matter, as suggested in thediscussion, unless otherwise specifically indicated.

What is claimed is:
 1. A method for synthesizing silver nanowires, themethod comprising: forming a reaction solution substantially free ofparamagnetic ions comprising a blend of polyol solvent, a polyvinylpyrrolidone capping agent, chloride anion, bromide anion, and afive-membered aromatic heterocyclic salt comprising an imidazolium,pyrazolium or thiazolium cation; heating the reaction solution to a peaktemperature; and adding a soluble silver salt.
 2. The method of claim 1,the imidazolium salt represented by the formula:

wherein: R₁ and R₂ are independently hydrogen, a linear, branched, orcyclic alkyl group, an unsaturated group, an arylalkyl group, with theproviso that R₁ and R₂ are not both hydrogen, and R₃ is a halogen or anysubstituent described for R₁ and R₂.
 3. The method of claim 1, thepyrazolium salt represented by the formula:

wherein: R₁ and R₂ are independently hydrogen, a linear, branched, orcyclic alkyl group, an unsaturated group, an arylalkyl group, with theproviso that R₁ and R₂ are not both hydrogen, and R₃ is a halogen or anysubstituent described for R₁ and R₂.
 4. The method of claim 1, thethiazolium salt represented by the formula:

wherein: R₁ and R₂ are independently hydrogen, a linear, branched, orcyclic alkyl group, an unsaturated group, an arylalkyl group, with theproviso that R₁ and R₂ are not both hydrogen, and R₃ is a halogen or anysubstituent described for R₁ and R₂.
 5. The method of claim 1 whereinmoles of the bromide ion are from about 0.6 to about 15 times moles ofthe chloride ion.
 6. The method of claim 1 wherein the reaction solutionis formed in a reactor, and a concentration based on added amounts intothe reactor for the heterocyclic cation is from about 0.00001M to about0.01M.
 7. The method of claim 1 wherein the reaction solution is formedin a reactor, and concentrations based on added amounts into the reactorare polyvinyl pyrrolidone capping agent 0.1 wt % to 10 wt %, bromideanion 0.00002M to 0.005M, chloride anion 0.00002M to 0.005M,five-membered aromatic heterocyclic salt 0.00002M to 0.005M, and solublesilver salt 0.0025M to 0.25M.
 8. The method of claim 1 wherein thepolyvinyl pyrrolidone capping agent comprises a blend of PVP K30 and PVPK90.
 9. The method of claim 1 wherein the peak temperature is at leastabout 145° C.
 10. The method of claim 1 wherein the soluble silver saltis added at a temperature within 5° C. of the peak temperature.
 11. Themethod of claim 1, further comprising purifying silver nanowires from acompleted reaction mixture, wherein the silver nanowires have an averagediameter of no more than about 18 nm and a standard deviation of thediameter of no more than about 2.5 nm.